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/completion.rs

@@ -31,6 +31,7 @@ use crate::mode::Mode;
 /// fragments the user thinks of as a single phrase:
 ///
 /// - `1:n` — the opener for `add 1:n relationship`.
+/// - `m:n` — the opener for `create m:n relationship` (ADR-0045).
 /// - `double precision` — the lone two-word SQL type alias
 ///   (ADR-0035 §6.3; the grammar has a dedicated branch so the per-word
 ///   `Ident` validator never has to make sense of `double` alone).
@@ -40,7 +41,7 @@ use crate::mode::Mode;
 ///   composite replaces the bare opener rather than appearing
 ///   alongside it.
 const COMPOSITE_CANDIDATES: &[(&str, &str)] =
-    &[("1", "1:n"), ("double", "double precision")];
+    &[("1", "1:n"), ("m", "m:n"), ("double", "double precision")];
 
 /// Per-project schema lookup cache (ADR-0022 §9, ADR-0024 §Phase D).
 ///
@@ -1346,12 +1347,19 @@ mod tests {
     fn at_token_boundary_offers_next_expected_keyword() {
         // After `create ` advanced mode offers `table` (valid in both
         // modes) plus the SQL-only `unique` (`create unique index`) and
-        // `index` — the shared-entry-word merge (ADR-0035 §4i d).
+        // `index` — the shared-entry-word merge (ADR-0035 §4i d) — and
+        // `m:n` (`create m:n relationship`, ADR-0045), surfaced as the
+        // composite (the bare `m` opener is filtered).
         // `table` (Both) blocks before the Advanced-only `unique`/`index`.
         let cs = cands("create ", 7);
         assert_eq!(
             cs,
-            vec!["table".to_string(), "unique".to_string(), "index".to_string()]
+            vec![
+                "table".to_string(),
+                "unique".to_string(),
+                "index".to_string(),
+                "m:n".to_string()
+            ]
         );
     }