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.

docs/requirements.md

@@ -276,9 +276,23 @@ since ADR-0027.)
   the same via drop + add today; one-step modify is a small
   follow-up using the existing rebuild-table machinery. ADR
   pending.
-- [ ] **C4** Convenience: `create m:n relationship from <T1> to
+- [x] **C4** Convenience: `create m:n relationship from <T1> to
   <T2>` produces an auto-named junction table the user can rename;
   pulls primary keys and FK definitions automatically.
+  *(Done 2026-06-10 via **ADR-0045**. `create m:n relationship from
+  <T1> to <T2> [as <name>]` builds 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`), available in both modes, compound-parent
+  PKs supported (ADR-0043). Self-referential m:n refused; PK-less parent
+  refused. Wired across every surface — completion (`m:n` composite),
+  hints, highlighting, `help`/usage, and the advanced-mode DSL→SQL
+  teaching echo (the generated `CREATE TABLE … FOREIGN KEY …`). 9
+  integration + 7 typing-surface + echo/parse unit tests. The build
+  surfaced — and fixed — two latent simple-mode dispatch/completion
+  assumptions ("≤1 DSL form per entry word"), now generalized
+  behaviour-preservingly.)*
 - [x] **C5** Data operations: insert / update / delete via DSL.
   *(ADR-0014. INSERT short and long forms, UPDATE/DELETE with
   required WHERE plus `--all-rows` opt-in, `show data <T>`,