ADR-0016 + Iter 5/6 follow-up: pretty table rendering

Replaces the placeholder pipe-and-dash output with Unicode
box-drawing tables for both data results and table-structure
listings, per ADR-0016.

* New `src/output_render.rs` module with `render_data_table`
  and `render_structure`. Hand-rolled to match the project's
  existing CSV/YAML pattern; ~300 lines.
* Header-only outer-frame border style: outer ┌─┐│└─┘ box +
  ├─┤ header underline, no per-row separators. NULL renders
  as `(null)`; cell newlines/tabs/control chars become
  `↵`/`→`/`·` as display-only substitutions.
* Type-aware column alignment: numeric types right-aligned,
  everything else left. `DataResult` gains a `column_types:
  Vec<Option<Type>>` field, populated from the existing
  metadata lookup at the two query sites in db.rs (no new
  query paths).
* Structure view shows Name | Type | Constraints columns;
  References / Referenced-by sections retain plain-text
  format, leaving room for the future relationship-rendering
  ADR.
* 18 new unit tests in output_render.rs (plus 4 insta
  snapshots for the canonical layouts). Existing assertions
  in app.rs and walking_skeleton.rs updated to match the new
  format.

Total: 426 passing, 0 failing, 0 skipped (up from 408).
Clippy clean.

src/app.rs

@@ -722,49 +722,8 @@ impl App {
         let summary = format!("[ok] {} {}", command.verb(), command.display_subject());
         self.note_system(summary);
         if let Some(desc) = description.as_ref() {
-            self.note_system(format!("  {}", desc.name));
-            for col in &desc.columns {
-                let pk = if col.primary_key { " [PK]" } else { "" };
-                let nn = if col.notnull { " NOT NULL" } else { "" };
-                // Prefer the user-facing type recovered from our
-                // metadata table; fall back to the SQLite type only
-                // if metadata is missing (only happens for tables we
-                // didn't create — not in the current flow).
-                let type_display = col
-                    .user_type
-                    .map_or_else(|| col.sqlite_type.to_lowercase(), |t| t.keyword().to_string());
-                self.note_system(format!(
-                    "    {} {}{}{}",
-                    col.name, type_display, pk, nn
-                ));
-            }
-            if !desc.outbound_relationships.is_empty() {
-                self.note_system("  References:");
-                for r in &desc.outbound_relationships {
-                    self.note_system(format!(
-                        "    {} → {}.{}  ({}, on delete {}, on update {})",
-                        r.local_column,
-                        r.other_table,
-                        r.other_column,
-                        r.name,
-                        r.on_delete,
-                        r.on_update,
-                    ));
-                }
-            }
-            if !desc.inbound_relationships.is_empty() {
-                self.note_system("  Referenced by:");
-                for r in &desc.inbound_relationships {
-                    self.note_system(format!(
-                        "    {}.{} → {}  ({}, on delete {}, on update {})",
-                        r.other_table,
-                        r.other_column,
-                        r.local_column,
-                        r.name,
-                        r.on_delete,
-                        r.on_update,
-                    ));
-                }
+            for line in crate::output_render::render_structure(desc) {
+                self.note_system(line);
             }
         }
         self.current_table = description;
@@ -773,7 +732,7 @@ impl App {
     fn handle_dsl_query_success(&mut self, command: &Command, data: &DataResult) {
         let summary = format!("[ok] {} {}", command.verb(), command.display_subject());
         self.note_system(summary);
-        for line in render_data_view(data) {
+        for line in crate::output_render::render_data_table(data) {
             self.note_system(line);
         }
     }
@@ -785,7 +744,7 @@ impl App {
             command.display_subject()
         ));
         self.note_system(format!("  {} row(s) inserted", result.rows_affected));
-        for line in render_data_view(&result.data) {
+        for line in crate::output_render::render_data_table(&result.data) {
             self.note_system(line);
         }
     }
@@ -797,7 +756,7 @@ impl App {
             command.display_subject()
         ));
         self.note_system(format!("  {} row(s) updated", result.rows_affected));
-        for line in render_data_view(&result.data) {
+        for line in crate::output_render::render_data_table(&result.data) {
             self.note_system(line);
         }
     }
@@ -1264,77 +1223,6 @@ fn render_cascade_effect(effect: &CascadeEffect) -> String {
     )
 }
 
-/// Render a data result as a sequence of aligned-column text
-/// lines suitable for the output panel. Pretty box-drawing
-/// rendering is V4 territory; this version uses simple
-/// pipe-and-dash separators.
-fn render_data_view(data: &DataResult) -> Vec<String> {
-    let header = data.columns.clone();
-    let body: Vec<Vec<String>> = data
-        .rows
-        .iter()
-        .map(|row| {
-            row.iter()
-                .map(|cell| {
-                    cell.as_ref()
-                        .map_or_else(|| "(null)".to_string(), Clone::clone)
-                })
-                .collect()
-        })
-        .collect();
-
-    // Column widths = max(header, all cells) per column.
-    let mut widths: Vec<usize> = header.iter().map(String::len).collect();
-    for row in &body {
-        for (i, cell) in row.iter().enumerate() {
-            if i < widths.len() && cell.chars().count() > widths[i] {
-                widths[i] = cell.chars().count();
-            }
-        }
-    }
-
-    let mut out: Vec<String> = Vec::with_capacity(body.len() + 3);
-    out.push(format!("  {}", join_padded(&header, &widths)));
-    out.push(format!("  {}", separator_row(&widths)));
-    if body.is_empty() {
-        out.push("  (no rows)".to_string());
-    } else {
-        for row in &body {
-            out.push(format!("  {}", join_padded(row, &widths)));
-        }
-    }
-    out
-}
-
-fn join_padded(cells: &[String], widths: &[usize]) -> String {
-    let mut s = String::new();
-    for (i, cell) in cells.iter().enumerate() {
-        if i > 0 {
-            s.push_str(" | ");
-        }
-        let w = widths.get(i).copied().unwrap_or(0);
-        s.push_str(cell);
-        let pad = w.saturating_sub(cell.chars().count());
-        for _ in 0..pad {
-            s.push(' ');
-        }
-    }
-    s
-}
-
-fn separator_row(widths: &[usize]) -> String {
-    let mut s = String::new();
-    for (i, w) in widths.iter().enumerate() {
-        if i > 0 {
-            s.push_str("-+-");
-        }
-        for _ in 0..*w {
-            s.push('-');
-        }
-    }
-    s
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1601,9 +1489,13 @@ mod tests {
             description: Some(desc.clone()),
         });
         assert_eq!(app.current_table, Some(desc));
-        let last = app.output.back().unwrap();
-        // Last line is the column row of the structure summary.
-        assert!(last.text.contains("id"));
+        // Some line in the output buffer is the structure
+        // table row that contains `id` (followed by border
+        // chars on either side).
+        assert!(
+            app.output.iter().any(|l| l.text.contains("id")),
+            "expected `id` somewhere in structure output",
+        );
         // Earlier line is the [ok] header.
         assert!(app.output.iter().any(|l| l.text.starts_with("[ok]")));
     }