feat: create m:n relationship convenience command (C4, ADR-0045)

`create m:n relationship from <T1> to <T2> [as <name>]` generates a
junction table with one FK column per parent PK column ({table}_{pkcol},
typed via fk_target_type), a compound PK over them, and two CASCADE 1:n
relationships -- all in one do_create_table call = one undo step.
Auto-named {T1}_{T2} (optional `as`), both modes, compound-parent PKs
supported (ADR-0043). Self-referential m:n / PK-less parent / internal
junction name / name collision all refused.

Wired across every surface: grammar (separate CREATE_M2N node), worker
executor, runtime dispatch, completion ("m:n" composite), hints,
highlighting, help + usage catalog + disambiguator, and the advanced-mode
DSL->SQL teaching echo (render_create_m2n, round-trips as valid SQL).

Generalized/fixed framework assumptions the build + two /runda passes
surfaced (all behaviour-preserving for existing commands):
- simple-mode dispatch committed simple.first() unconditionally -> tries
  candidates, so `create table` no longer shadows `create m:n`.
- the completion continuation-merge was advanced-only -> runs in simple
  mode too when an entry word has >1 DSL form (gated simple_count>1).
- do_create_table now rejects internal `__rdbms_*` names (closes a
  pre-existing hole on the DSL create-table path too, not just m:n).
- usage disambiguator now recognizes the `m:n` opener.

Tests: 14 integration (tests/it/m2n.rs), 7 typing-surface matrix, echo /
highlight / usage / internal-name units. Closes C4.
2237 pass / 0 fail / 1 ignored. Clippy clean.

src/dsl/walker/highlight.rs

@@ -211,6 +211,21 @@ mod tests {
         assert_eq!(run("quit"), vec![(0, 4, HighlightClass::Keyword)]);
     }
 
+    #[test]
+    fn create_m2n_relationship_highlights_cleanly() {
+        // ADR-0045: a valid `create m:n relationship` line classifies
+        // with no Error runs; keywords are keywords and the table names
+        // are identifiers (the `m:n` opener is a Literal, keyword-classed).
+        let runs = run("create m:n relationship from A to B");
+        assert!(
+            !runs.iter().any(|(_, _, c)| *c == HighlightClass::Error),
+            "no Error highlight on a valid m:n line: {runs:?}"
+        );
+        let kinds: Vec<HighlightClass> = runs.iter().map(|(_, _, c)| *c).collect();
+        assert!(kinds.contains(&HighlightClass::Keyword), "keywords highlighted: {runs:?}");
+        assert!(kinds.contains(&HighlightClass::Identifier), "table names highlighted: {runs:?}");
+    }
+
     #[test]
     fn keyword_plus_identifier_via_walker() {
         // `show data Customers` walks end-to-end.