feat: support explain over advanced-mode SQL queries

explain now wraps the advanced SQL commands — select, with (CTE),
insert, update, delete — in addition to the DSL show data/update/
delete it already covered, rendering through the same plan tree
(ADR-0039, closing the ADR-0030 OOS-2 gap).

Implemented as a second Advanced `explain` CommandNode under the
shared entry word, reusing the established shared-word dispatch
(SQL-first, DSL-fallback) rather than new grammar machinery.
build_explain_sql slices the inner SQL off the source and reuses the
existing SQL builders; do_explain_plan runs EXPLAIN QUERY PLAN over
the carried text verbatim (never executes, so safe for destructive
verbs). Advanced explain update/delete now route through SQL with an
identical plan; DSL-explain tests pinned to simple mode. Help and
usage text now list the advanced explain forms.

src/dsl/grammar/mod.rs

@@ -634,6 +634,13 @@ pub static REGISTRY: &[(&CommandNode, CommandCategory)] = &[
     (&data::SQL_INSERT, CommandCategory::Advanced),
     (&data::SQL_UPDATE, CommandCategory::Advanced),
     (&data::SQL_DELETE, CommandCategory::Advanced),
+    // Shared entry word `explain` (ADR-0039): the `Simple` DSL
+    // `data::EXPLAIN` (above) wraps `show data` / `update` / `delete`;
+    // this `Advanced` node wraps the SQL `select` / `with` / `insert`
+    // / `update` / `delete`. SQL-first / DSL-fallback in advanced mode
+    // (so `explain show data …` and DSL-only `--all-rows` still reach
+    // the DSL node); DSL-only in simple mode.
+    (&data::EXPLAIN_SQL, CommandCategory::Advanced),
     // Shared entry word `create` (ADR-0035 §2): the simple
     // `ddl::CREATE` (above) and these advanced SQL nodes. The
     // dispatcher tries the advanced candidates first in advanced mode
@@ -782,4 +789,22 @@ mod usage_key_tests {
             Some("parse.usage.create_table"),
         );
     }
+
+    #[test]
+    fn no_two_registered_commands_share_a_help_id() {
+        // `note_help` emits one help block per `help_id: Some(_)`
+        // with no dedup, so a duplicate help_id prints the same
+        // command twice in `help`. Shared-entry-word `Advanced`
+        // nodes (SQL_INSERT, …, EXPLAIN_SQL) therefore carry
+        // `help_id: None` and defer to their `Simple` sibling.
+        let mut seen = std::collections::HashSet::new();
+        for (command, _category) in super::REGISTRY {
+            if let Some(id) = command.help_id {
+                assert!(
+                    seen.insert(id),
+                    "duplicate help_id `{id}` in REGISTRY would print twice in `help`",
+                );
+            }
+        }
+    }
 }