DSL parser, async DB worker, types, history, metadata, polish

Track 1 implementation plus polish round.

Parser (chumsky):
- Grammar-based DSL producing a typed Command AST.
- create table X with pk [name:type[,name:type...]] supports
  arbitrary names, any user type, compound PKs natively. Bare
  form errors with a friendly hint pointing at `with pk`.
- add column to table X: Name (type); drop table X.
- Required clauses use keyword grammar; -- reserved for opt-in
  flags (ADR-0009). Custom Rich reasons preferred when surfacing
  chumsky errors so unknown-type messages list valid alternatives.

Database (ADR-0010, ADR-0012):
- rusqlite + STRICT tables + foreign_keys=ON.
- Dedicated worker thread; mpsc Request inbox, oneshot replies.
- Typed DbError with friendly_message() hook for H1.
- Internal __rdbms_playground_columns metadata table preserves
  user-facing types across schema reads, atomically maintained
  alongside DDL via Connection transactions. list_tables hides
  it via the new __rdbms_ internal-table convention.

Types (ADR-0005, ADR-0011):
- All ten user-facing types: text, int, real, decimal, bool,
  date, datetime, blob, serial, shortid.
- Type::fk_target_type() for FK-side column-type rule
  (Serial->Int, ShortId->Text, others identity) -- foundation
  for the FK iteration.

App / Runtime / UI:
- update() stays pure-sync; runtime dispatches DSL via spawned
  tasks, results post back as AppEvent::Dsl*.
- Items panel renders live tables list; output panel shows the
  user-facing structure of the current table after each DDL.
- In-memory command history (Up/Down, draft preservation,
  consecutive-duplicate dedup) -- I2 partial.
- Mouse capture removed; terminal native text selection
  restored (toggle approach revisited when scroll/click
  features land).

Docs:
- ADRs 0009 (DSL syntax conventions), 0010 (DB worker),
  0011 (FK type compat), 0012 (internal metadata table).
- requirements.md progress notes; new V4 entry for the
  scrollable session-log + inline rich rendering + Markdown
  export direction.

Tests: 103 passing (91 lib + 12 integration), 0 skipped.
Clippy clean with nursery enabled.

docs/adr/README.md

@@ -14,3 +14,7 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0006 — Undo snapshots and replay log](0006-undo-snapshots-and-replay-log.md)
 - [ADR-0007 — Sharing and export](0007-sharing-and-export.md)
 - [ADR-0008 — Testing approach](0008-testing-approach.md)
+- [ADR-0009 — DSL command syntax conventions](0009-dsl-command-syntax-conventions.md)
+- [ADR-0010 — Database access via a dedicated worker thread](0010-database-access-via-worker-thread.md)
+- [ADR-0011 — Foreign-key column type compatibility](0011-fk-column-type-compatibility.md)
+- [ADR-0012 — Internal metadata for user-facing column types](0012-internal-metadata-for-user-facing-types.md)