explain: styled plan tree + annotation taxonomy (ADR-0028 step 4)

`render_explain_plan` now classifies each plan node and colours
its category-bearing keywords through the styled-runs mechanism.

- `PLAN_TAXONOMY`: a substring-pattern table mapping the
  engine's plan vocabulary to four semantic classes — full
  scan / temp B-tree -> Expensive, index search / covering
  index / PK lookup -> Efficient, automatic index ->
  AutomaticIndex. An unrecognised detail renders neutral, since
  the engine's plan vocabulary may grow.
- Only the matched keyword run carries the category colour;
  connectors, prefixes and table / index names stay neutral
  (ADR-0028 §6). The display-SQL line is wholly neutral.
- An automatic-index node also gets the distinct "← add an
  index?" advice tag, so it reads as guidance, not merely
  "this is slow".

1158 tests pass (+7); clippy clean.

src/output_render.rs

@@ -22,7 +22,7 @@
 
 use std::collections::HashSet;
 
-use crate::app::{OutputKind, OutputLine};
+use crate::app::{OutputKind, OutputLine, OutputSpan, OutputStyleClass};
 use crate::db::{ColumnDescription, DataResult, ExplainRow, QueryPlan, TableDescription};
 use crate::dsl::Type;
 use crate::mode::Mode;
@@ -152,13 +152,45 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
     out
 }
 
+/// The plan annotation taxonomy (ADR-0028 §4).
+///
+/// A `detail` string is classified by the **first** entry
+/// whose marker substring it contains; the marker's byte range
+/// is also the run that carries the category colour (ADR-0028
+/// §6 — "only the category-bearing keywords"). Order matters:
+/// the automatic-index markers come before the plain-index and
+/// `SCAN` ones so a `SEARCH … USING AUTOMATIC … INDEX` is not
+/// mis-read as a plain index search or a full scan; `SCAN`
+/// comes last so a covering-index scan (`SCAN … USING INDEX …`)
+/// classifies on its index, not on the `SCAN`.
+///
+/// A `detail` matching no marker renders neutral — the engine's
+/// plan vocabulary may grow (ADR-0028 §4).
+const PLAN_TAXONOMY: &[(&str, OutputStyleClass)] = &[
+    ("USING AUTOMATIC COVERING INDEX", OutputStyleClass::AutomaticIndex),
+    ("USING AUTOMATIC INDEX", OutputStyleClass::AutomaticIndex),
+    ("USING COVERING INDEX", OutputStyleClass::Efficient),
+    ("USING INTEGER PRIMARY KEY", OutputStyleClass::Efficient),
+    ("USING PRIMARY KEY", OutputStyleClass::Efficient),
+    ("USING INDEX", OutputStyleClass::Efficient),
+    ("USE TEMP B-TREE", OutputStyleClass::Expensive),
+    ("SCAN", OutputStyleClass::Expensive),
+];
+
+/// The short "you should add an index here" tag appended to an
+/// automatic-index plan node (ADR-0028 §6 — the distinct
+/// marker that makes it read as advice, not merely "slow").
+const AUTO_INDEX_TAG: &str = "  ← add an index?";
+
 /// Render a captured query plan as a box-drawing tree
-/// (ADR-0028 §3).
+/// (ADR-0028 §3/§4).
 ///
 /// The standard-SQL display form of the explained statement is
 /// shown first; the plan tree follows, built from each row's
 /// `id` / `parent` links. Node text is the engine's `detail`
-/// string **verbatim** — nothing is reworded.
+/// string **verbatim** — nothing is reworded — but each
+/// node's category-bearing keywords carry a semantic colour
+/// from the annotation taxonomy.
 ///
 /// Unlike the `Vec<String>`-returning renderers above this
 /// returns ready-built `OutputLine`s, because a plan line
@@ -167,7 +199,8 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
 #[must_use]
 pub fn render_explain_plan(plan: &QueryPlan, mode: Mode) -> Vec<OutputLine> {
     let mut out: Vec<OutputLine> = Vec::with_capacity(plan.rows.len() + 1);
-    out.push(plain_plan_line(plan.display_sql.clone(), mode));
+    // The display SQL is wholly neutral — no category keywords.
+    out.push(neutral_plan_line(plan.display_sql.clone(), mode));
     // `emitted` guards against a malformed plan with a cyclic
     // or self-referential `parent` link — every node is drawn
     // at most once.
@@ -196,25 +229,73 @@ fn render_plan_subtree(
         }
         let is_last = idx == last_idx;
         let connector = if is_last { "└─ " } else { "├─ " };
-        out.push(plain_plan_line(
-            format!("{prefix}{connector}{}", row.detail),
-            mode,
-        ));
+        out.push(plan_node_line(prefix, connector, &row.detail, mode));
         let child_prefix =
             format!("{prefix}{}", if is_last { "   " } else { "│  " });
         render_plan_subtree(rows, row.id, &child_prefix, out, emitted, mode);
     }
 }
 
-/// A plain (unstyled) plan output line. ADR-0028 step 4 swaps
-/// the tree lines for span-styled ones; the display-SQL line
-/// stays plain.
-const fn plain_plan_line(text: String, mode: Mode) -> OutputLine {
-    OutputLine {
-        text,
-        kind: OutputKind::System,
-        mode_at_submission: mode,
-        styled_runs: None,
+/// Classify `detail` against the taxonomy, returning the byte
+/// offset + length of the matched marker and its colour class.
+/// `None` when no marker matches (a neutral node).
+fn classify_detail(detail: &str) -> Option<(usize, usize, OutputStyleClass)> {
+    PLAN_TAXONOMY
+        .iter()
+        .find_map(|(marker, class)| detail.find(marker).map(|off| (off, marker.len(), *class)))
+}
+
+/// Build one plan-tree node line: `{prefix}{connector}{detail}`
+/// with the connectors / prefix / table & index names neutral
+/// and only the category-bearing keyword run coloured
+/// (ADR-0028 §6). An automatic-index node also gets the
+/// "add an index?" advice tag.
+fn plan_node_line(prefix: &str, connector: &str, detail: &str, mode: Mode) -> OutputLine {
+    let mut text = format!("{prefix}{connector}{detail}");
+    let detail_start = prefix.len() + connector.len();
+    let mut runs: Vec<OutputSpan> = Vec::new();
+    match classify_detail(detail) {
+        Some((offset, len, class)) => {
+            let marker_start = detail_start + offset;
+            let marker_end = marker_start + len;
+            push_neutral(&mut runs, 0, marker_start);
+            runs.push(OutputSpan {
+                byte_range: (marker_start, marker_end),
+                class,
+            });
+            push_neutral(&mut runs, marker_end, text.len());
+            if class == OutputStyleClass::AutomaticIndex {
+                let tag_start = text.len();
+                text.push_str(AUTO_INDEX_TAG);
+                runs.push(OutputSpan {
+                    byte_range: (tag_start, text.len()),
+                    class,
+                });
+            }
+        }
+        None => push_neutral(&mut runs, 0, text.len()),
+    }
+    OutputLine::styled(text, OutputKind::System, mode, runs)
+}
+
+/// A wholly-neutral plan line — used for the display-SQL line,
+/// which carries no category keywords. Styled (rather than
+/// plain) so it renders in the neutral foreground colour
+/// instead of the whole-line `System` styling (ADR-0028 §6).
+fn neutral_plan_line(text: String, mode: Mode) -> OutputLine {
+    let mut runs = Vec::new();
+    push_neutral(&mut runs, 0, text.len());
+    OutputLine::styled(text, OutputKind::System, mode, runs)
+}
+
+/// Push a `Neutral` span covering `[start, end)`, skipping the
+/// empty case.
+fn push_neutral(runs: &mut Vec<OutputSpan>, start: usize, end: usize) {
+    if end > start {
+        runs.push(OutputSpan {
+            byte_range: (start, end),
+            class: OutputStyleClass::Neutral,
+        });
     }
 }
 
@@ -757,6 +838,119 @@ mod tests {
         assert!(lines[2].text.starts_with("   ├─ child-a"));
     }
 
+    /// The semantic class of the styled run covering the first
+    /// occurrence of `needle` in `line`'s text.
+    fn span_class_for(line: &OutputLine, needle: &str) -> OutputStyleClass {
+        let runs = line.styled_runs.as_ref().expect("line should be styled");
+        let at = line.text.find(needle).expect("needle present in text");
+        runs.iter()
+            .find(|s| s.byte_range.0 <= at && at < s.byte_range.1)
+            .map(|s| s.class)
+            .expect("a styled run covers the needle")
+    }
+
+    fn one_node_plan(detail: &str) -> QueryPlan {
+        QueryPlan {
+            display_sql: "SELECT 1".to_string(),
+            rows: vec![ExplainRow {
+                id: 1,
+                parent: 0,
+                detail: detail.to_string(),
+            }],
+        }
+    }
+
+    #[test]
+    fn render_explain_plan_colours_a_full_scan_expensive() {
+        let plan = one_node_plan("SCAN Customers");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        assert_eq!(span_class_for(&lines[1], "SCAN"), OutputStyleClass::Expensive);
+        // The table name stays neutral (ADR-0028 §6).
+        assert_eq!(
+            span_class_for(&lines[1], "Customers"),
+            OutputStyleClass::Neutral,
+        );
+    }
+
+    #[test]
+    fn render_explain_plan_colours_an_index_search_efficient() {
+        let plan = one_node_plan("SEARCH Customers USING INDEX Customers_Email_idx (Email=?)");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        assert_eq!(
+            span_class_for(&lines[1], "USING INDEX"),
+            OutputStyleClass::Efficient,
+        );
+        // The index name and the connector stay neutral.
+        assert_eq!(
+            span_class_for(&lines[1], "Customers_Email_idx"),
+            OutputStyleClass::Neutral,
+        );
+        assert_eq!(span_class_for(&lines[1], "└─"), OutputStyleClass::Neutral);
+    }
+
+    #[test]
+    fn render_explain_plan_flags_an_automatic_index() {
+        let plan =
+            one_node_plan("SEARCH Orders USING AUTOMATIC COVERING INDEX (CustId=?)");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        assert_eq!(
+            span_class_for(&lines[1], "USING AUTOMATIC COVERING INDEX"),
+            OutputStyleClass::AutomaticIndex,
+        );
+        // ADR-0028 §6: the distinct "add an index" advice tag.
+        assert!(
+            lines[1].text.ends_with("← add an index?"),
+            "got {:?}",
+            lines[1].text,
+        );
+        assert_eq!(
+            span_class_for(&lines[1], "add an index"),
+            OutputStyleClass::AutomaticIndex,
+        );
+    }
+
+    #[test]
+    fn render_explain_plan_temp_btree_is_expensive() {
+        let plan = one_node_plan("USE TEMP B-TREE FOR ORDER BY");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        assert_eq!(
+            span_class_for(&lines[1], "USE TEMP B-TREE"),
+            OutputStyleClass::Expensive,
+        );
+    }
+
+    #[test]
+    fn render_explain_plan_covering_index_scan_classifies_on_the_index() {
+        // A `SCAN … USING INDEX …` is a covering-index scan —
+        // it must read as efficient, not as a full scan.
+        let plan = one_node_plan("SCAN Customers USING COVERING INDEX idx");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        assert_eq!(
+            span_class_for(&lines[1], "USING COVERING INDEX"),
+            OutputStyleClass::Efficient,
+        );
+        assert_eq!(span_class_for(&lines[1], "SCAN"), OutputStyleClass::Neutral);
+    }
+
+    #[test]
+    fn render_explain_plan_unrecognised_detail_is_neutral() {
+        let plan = one_node_plan("CO-ROUTINE 0x1");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        let runs = lines[1].styled_runs.as_ref().unwrap();
+        assert!(
+            runs.iter().all(|s| s.class == OutputStyleClass::Neutral),
+            "an unrecognised detail must render wholly neutral: {runs:?}",
+        );
+    }
+
+    #[test]
+    fn render_explain_plan_display_sql_line_is_neutral() {
+        let plan = one_node_plan("SCAN T");
+        let lines = render_explain_plan(&plan, Mode::Simple);
+        let runs = lines[0].styled_runs.as_ref().unwrap();
+        assert!(runs.iter().all(|s| s.class == OutputStyleClass::Neutral));
+    }
+
     #[test]
     fn render_explain_plan_survives_a_self_referential_node() {
         // A malformed plan node that is both a root (`parent ==