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/app.rs

@@ -398,6 +398,10 @@ impl App {
                 self.handle_dsl_query_success(&command, &data);
                 Vec::new()
             }
+            AppEvent::DslExplainSucceeded { command, plan } => {
+                self.handle_dsl_explain_success(&command, &plan);
+                Vec::new()
+            }
             AppEvent::DslInsertSucceeded { command, result } => {
                 self.handle_dsl_insert_success(&command, &result);
                 Vec::new()
@@ -1201,6 +1205,20 @@ impl App {
         }
     }
 
+    fn handle_dsl_explain_success(
+        &mut self,
+        command: &Command,
+        plan: &crate::db::QueryPlan,
+    ) {
+        self.note_ok_summary(command);
+        // ADR-0028 §3: the display SQL, then the plan tree.
+        // `render_explain_plan` returns ready-built `OutputLine`s
+        // so it can carry the per-span styling (ADR-0028 §5).
+        for line in crate::output_render::render_explain_plan(plan, self.mode) {
+            self.push_output(line);
+        }
+    }
+
     fn handle_dsl_insert_success(&mut self, command: &Command, result: &InsertResult) {
         self.note_ok_summary(command);
         self.note_system(crate::t!("ok.rows_inserted", count = result.rows_affected));
@@ -1416,6 +1434,11 @@ impl App {
                 (Operation::Query, Some(name.as_str()), None)
             }
             C::Replay { .. } => (Operation::Replay, None, None),
+            // An `explain` failure (e.g. unknown table) is best
+            // described by the wrapped query it failed to plan.
+            C::Explain { query } => {
+                return self.build_translate_context(query, facts);
+            }
             // App-lifecycle commands never reach this path —
             // `dispatch_input` routes them through
             // `dispatch_app_command` before the DSL execution
@@ -2419,6 +2442,47 @@ mod tests {
         assert!(app.output.iter().any(|l| l.text.starts_with("[ok]")));
     }
 
+    #[test]
+    fn explain_success_event_renders_display_sql_and_plan_tree() {
+        let mut app = App::new();
+        let cmd = Command::Explain {
+            query: Box::new(Command::ShowData {
+                name: "Customers".to_string(),
+                filter: None,
+                limit: None,
+            }),
+        };
+        let plan = crate::db::QueryPlan {
+            display_sql: "SELECT \"id\" FROM \"Customers\"".to_string(),
+            rows: vec![crate::db::ExplainRow {
+                id: 2,
+                parent: 0,
+                detail: "SCAN Customers".to_string(),
+            }],
+        };
+        app.update(AppEvent::DslExplainSucceeded {
+            command: cmd,
+            plan,
+        });
+        // `[ok] explain Customers` header.
+        assert!(
+            app.output.iter().any(|l| l.text.starts_with("[ok]")
+                && l.text.contains("explain")),
+            "expected an [ok] explain header",
+        );
+        // The display SQL and the plan node both reach output.
+        assert!(
+            app.output
+                .iter()
+                .any(|l| l.text.contains("SELECT \"id\" FROM \"Customers\"")),
+            "expected the display SQL line",
+        );
+        assert!(
+            app.output.iter().any(|l| l.text.contains("SCAN Customers")),
+            "expected the plan-tree node",
+        );
+    }
+
     #[test]
     fn replay_command_dispatches_replay_action_not_execute_dsl() {
         // Submitting `replay <path>` must NOT produce an