Foreign-key relationships, rebuild-table, polish round

DSL:
- add 1:n relationship [as <name>] from <P>.<col> to <C>.<col>
  [on delete <action>] [on update <action>] [--create-fk]
- drop relationship <name> | from <P>.<col> to <C>.<col>
- show table <name> for re-displaying a structure on demand

Database (ADR-0013):
- Rebuild-table primitive following SQLite's
  ALTER-via-rebuild recipe (foreign_keys=OFF outside tx,
  copy-by-name, foreign_key_check before commit). Reusable for
  B2 (column drops/renames/type changes).
- ReferentialAction enum (no action / restrict / set null /
  cascade); SET DEFAULT awaits column DEFAULTs.
- __rdbms_playground_relationships metadata table -- names,
  auto-generated as <Parent>_<pcol>_to_<Child>_<ccol>.
- Type::fk_target_type() validation at declaration; friendly
  errors for type mismatch, non-PK target, missing column,
  duplicate name.
- describe_table populates symmetric outbound + inbound
  relationship lists. drop_table refuses while inbound
  references exist; outbound metadata cleaned up alongside drop.

App / UI:
- In-line cursor editing in the input field: Left, Right,
  Home, End, Delete, Backspace honoring UTF-8 boundaries.
- PageUp / PageDown scrolls the output buffer; viewport row
  count fed back from the renderer via App::note_output_viewport
  so scroll is capped against the actual visible area
  (regression-tested) and snaps to the bottom on new output.
- Failure messages quote the command portion ("verb target"
  failed: ...) for visual clarity; RelationshipSelector has a
  proper Display impl so "no such relationship" reads cleanly.
- Structure rendering shows References / Referenced by sections.

Docs:
- ADR-0013 covers naming, metadata table, symmetric view, and
  the rebuild-table strategy.
- requirements.md updates: C3 (FK done), B2 (primitive in),
  T3 (compound-PK FK still pending). New entries: I1a (cursor
  editing -- landed), I1b (Ctrl-A/E and readline shortcuts --
  pending), V4 partial scroll, V5 (show family), C3a (modify
  relationship -- deferred).

Tests: 154 passing (140 lib + 14 integration), 0 skipped.
Clippy clean with nursery enabled.

docs/adr/README.md

@@ -18,3 +18,4 @@ This directory contains the project's ADRs, recorded per
 - [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)
+- [ADR-0013 — Relationships, naming, and the rebuild-table strategy](0013-relationships-and-rebuild-table.md)