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/event.rs

@@ -9,7 +9,7 @@ use crossterm::event::KeyEvent;
 
 use crate::db::{
     AddColumnResult, ChangeColumnTypeResult, DataResult, DbError, DeleteResult,
-    DropColumnResult, InsertResult, TableDescription, UpdateResult,
+    DropColumnResult, InsertResult, QueryPlan, TableDescription, UpdateResult,
 };
 use crate::dsl::Command;
 
@@ -30,6 +30,9 @@ pub enum AppEvent {
     },
     /// A `show data` query succeeded.
     DslDataSucceeded { command: Command, data: DataResult },
+    /// An `explain …` command succeeded (ADR-0028). `plan`
+    /// carries the captured query plan; nothing was executed.
+    DslExplainSucceeded { command: Command, plan: QueryPlan },
     DslInsertSucceeded {
         command: Command,
         result: InsertResult,