explain: explain command end to end (ADR-0028 steps 2–3)

Add the `explain` prefix command — `explain show data`,
`explain update`, `explain delete` — from grammar through to a
rendered plan tree.

- Grammar: an `EXPLAIN` CommandNode whose shape is a Choice over
  the three explainable query shapes, referenced (not
  duplicated) through `Subgrammar`. `Command::Explain { query:
  Box<Self> }`; `build_show_data` is extracted so the role-based
  builders serve both standalone and explain-wrapped commands.
- Worker: SQL construction is split out of do_query_data /
  do_update / do_delete into `build_*_sql`, so EXPLAIN QUERY
  PLAN runs the exact same statement. `Request::ExplainPlan` /
  `do_explain_plan` capture the plan; `QueryPlan` / `ExplainRow`
  carry it back. EXPLAIN QUERY PLAN never executes, so
  explaining update/delete changes nothing.
- Display SQL: the executed statement with `?N` parameters
  inlined as standard-SQL literals via a quote-aware scan.
- Render: `render_explain_plan` draws the box-drawing plan tree
  (plain output; ADR-0028 step 4 adds the styled tree).
- Catalog: `parse.usage.explain` and the `help.data.explain`
  entry, so `explain` shows up in the in-app `help` listing.

1151 tests pass (+18); clippy clean.

src/dsl/grammar/data.rs

@@ -310,6 +310,48 @@ const DELETE_NODES: &[Node] = &[
 ];
 const DELETE_SHAPE: Node = Node::Seq(DELETE_NODES);
 
+// =================================================================
+// explain — `explain (show data … | update … | delete from …)`
+// =================================================================
+//
+// ADR-0028 §1: `explain` is a top-level command whose shape is a
+// `Choice` over the three explainable query commands. The inner
+// query grammars are *referenced* through `Subgrammar`, not
+// duplicated — so an explained command is parsed, completed,
+// hinted and highlighted exactly as it is on its own.
+//
+// `Subgrammar` needs a `&'static Node`; `SHOW_DATA` /
+// `UPDATE_SHAPE` / `DELETE_SHAPE` are `const` (and cannot be
+// referenced as `&'static`). These three thin `static` wrappers
+// over the existing `_NODES` slices give the references without
+// any churn to the standalone command shapes. `explain show`
+// references `EXPLAIN_SHOW_DATA` directly (not the `show`
+// command's `data | table` choice) — `explain` covers `show
+// data` only (ADR-0028 §1).
+
+static EXPLAIN_SHOW_DATA: Node = Node::Seq(SHOW_DATA_NODES);
+static EXPLAIN_UPDATE: Node = Node::Seq(UPDATE_NODES);
+static EXPLAIN_DELETE: Node = Node::Seq(DELETE_NODES);
+
+const EXPLAIN_SHOW_NODES: &[Node] = &[
+    Node::Word(Word::keyword("show")),
+    Node::Subgrammar(&EXPLAIN_SHOW_DATA),
+];
+const EXPLAIN_UPDATE_NODES: &[Node] = &[
+    Node::Word(Word::keyword("update")),
+    Node::Subgrammar(&EXPLAIN_UPDATE),
+];
+const EXPLAIN_DELETE_NODES: &[Node] = &[
+    Node::Word(Word::keyword("delete")),
+    Node::Subgrammar(&EXPLAIN_DELETE),
+];
+const EXPLAIN_CHOICES: &[Node] = &[
+    Node::Seq(EXPLAIN_SHOW_NODES),
+    Node::Seq(EXPLAIN_UPDATE_NODES),
+    Node::Seq(EXPLAIN_DELETE_NODES),
+];
+const EXPLAIN_SHAPE: Node = Node::Choice(EXPLAIN_CHOICES);
+
 // =================================================================
 // AST builders
 // =================================================================
@@ -355,11 +397,7 @@ fn build_show(path: &MatchedPath) -> Result<Command, ValidationError> {
         .nth(1);
     let name = require_ident(path, "table_name")?;
     match sub {
-        Some("data") => Ok(Command::ShowData {
-            name,
-            filter: build_show_filter(path)?,
-            limit: build_show_limit(path)?,
-        }),
+        Some("data") => build_show_data(path),
         Some("table") => Ok(Command::ShowTable { name }),
         _ => Err(ValidationError {
             message_key: "parse.error_wrapper",
@@ -368,6 +406,18 @@ fn build_show(path: &MatchedPath) -> Result<Command, ValidationError> {
     }
 }
 
+/// Build a `show data` command from a matched path. Role-based
+/// (no positional `nth` lookups), so it serves both the
+/// standalone `show data` entry word and the `explain show
+/// data …` wrapper, where the entry-word offset shifts.
+fn build_show_data(path: &MatchedPath) -> Result<Command, ValidationError> {
+    Ok(Command::ShowData {
+        name: require_ident(path, "table_name")?,
+        filter: build_show_filter(path)?,
+        limit: build_show_limit(path)?,
+    })
+}
+
 /// The optional `where <expr>` of a `show data`. The expression
 /// terminals run from just past `Word("where")` to the start of
 /// the `limit` clause (or the end of the path) — neither the
@@ -676,6 +726,39 @@ fn build_delete(path: &MatchedPath) -> Result<Command, ValidationError> {
     Ok(Command::Delete { table, filter })
 }
 
+/// Build `Command::Explain` (ADR-0028 §1). The matched-word
+/// sequence is `[explain, show|update|delete, …]` — the entry
+/// word `explain` is at index 0, the inner command's lead word
+/// at index 1. The inner command is built by the same builder
+/// it uses standalone (`build_show_data` / `build_update` /
+/// `build_delete`), all of which are role-based and so are
+/// indifferent to the entry-word offset the `explain` prefix
+/// introduces.
+fn build_explain(path: &MatchedPath) -> Result<Command, ValidationError> {
+    let inner_word = path
+        .items
+        .iter()
+        .filter_map(|i| match &i.kind {
+            MatchedKind::Word(w) => Some(*w),
+            _ => None,
+        })
+        .nth(1);
+    let inner = match inner_word {
+        Some("show") => build_show_data(path)?,
+        Some("update") => build_update(path)?,
+        Some("delete") => build_delete(path)?,
+        _ => {
+            return Err(ValidationError {
+                message_key: "parse.error_wrapper",
+                args: vec![("detail", "unknown explain target".to_string())],
+            });
+        }
+    };
+    Ok(Command::Explain {
+        query: Box::new(inner),
+    })
+}
+
 // =================================================================
 // replay — `replay <bare-path>` | `replay '<path>'`
 // =================================================================
@@ -744,3 +827,85 @@ pub static REPLAY: CommandNode = CommandNode {
     ast_builder: build_replay,
     help_id: Some("data.replay"),
     usage_ids: &["parse.usage.replay"],};
+
+pub static EXPLAIN: CommandNode = CommandNode {
+    entry: Word::keyword("explain"),
+    shape: EXPLAIN_SHAPE,
+    ast_builder: build_explain,
+    help_id: Some("data.explain"),
+    usage_ids: &["parse.usage.explain"],};
+
+// =================================================================
+// Tests — `explain` grammar (ADR-0028 §1)
+// =================================================================
+
+#[cfg(test)]
+mod explain_tests {
+    use super::Command;
+    use crate::dsl::parser::parse_command;
+
+    /// Parse `input` and unwrap the `Command::Explain` wrapper,
+    /// returning the inner command.
+    fn explain_inner(input: &str) -> Command {
+        match parse_command(input).expect("explain should parse") {
+            Command::Explain { query } => *query,
+            other => panic!("expected Command::Explain, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn explain_show_data_wraps_a_show_data() {
+        assert!(matches!(
+            explain_inner("explain show data Customers"),
+            Command::ShowData { .. }
+        ));
+    }
+
+    #[test]
+    fn explain_show_data_carries_where_and_limit_through() {
+        match explain_inner("explain show data Customers where id = 1 limit 5") {
+            Command::ShowData { name, filter, limit } => {
+                assert_eq!(name, "Customers");
+                assert!(filter.is_some(), "where clause should survive");
+                assert_eq!(limit, Some(5));
+            }
+            other => panic!("expected ShowData, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn explain_update_wraps_an_update() {
+        assert!(matches!(
+            explain_inner("explain update Customers set Name='Bo' where id=1"),
+            Command::Update { .. }
+        ));
+    }
+
+    #[test]
+    fn explain_delete_wraps_a_delete() {
+        assert!(matches!(
+            explain_inner("explain delete from Customers where id=1"),
+            Command::Delete { .. }
+        ));
+    }
+
+    #[test]
+    fn explain_of_an_incomplete_update_is_a_parse_error() {
+        // A bare `update` still needs its `where` / `--all-rows`
+        // (ADR-0028 §1: `explain` of an incomplete command is the
+        // same parse error the command alone would be).
+        assert!(parse_command("explain update Customers set Name='Bo'").is_err());
+    }
+
+    #[test]
+    fn explain_does_not_cover_show_table() {
+        // `explain` covers `show data` only (ADR-0028 §1).
+        assert!(parse_command("explain show table Customers").is_err());
+    }
+
+    #[test]
+    fn bare_explain_is_a_parse_error() {
+        assert!(parse_command("explain").is_err());
+        assert!(parse_command("explain show").is_err());
+    }
+}