docs: handoff 23 — ADR-0029 complete; tick C3

ADR-0029 (column constraints — NOT NULL / UNIQUE / CHECK /
DEFAULT) is fully implemented across the handoff-22 and
handoff-23 sessions. Ticks requirement C3, and corrects
ADR §10's CHECK-error wording to the compiled-SQL form per
the §7 storage deviation.

docs/requirements.md

@@ -146,17 +146,21 @@ handoff-14 cleanup; 449 after B2/C2.)
   refused with friendly messages (drop the relationship
   first); SQLite STRICT enforces type compatibility on the
   data copy during a type change.
-- [ ] **C3** Schema constraints: primary key (single and
+- [x] **C3** Schema constraints: primary key (single and
   compound), foreign key with `ON DELETE` / `ON UPDATE` referential
   actions, indexes, `NOT NULL`, `UNIQUE`, `CHECK`, `DEFAULT`.
-  *(Progress: PK including compound done at create-table time;
+  *(PK including compound done at create-table time;
   FK with `ON DELETE` / `ON UPDATE` actions done (ADR-0013) —
   declared via `add 1:n relationship`; symmetric outbound +
   inbound view in the structure renderer; type compatibility
   validated at declaration via `Type::fk_target_type()`.
   Indexes done (ADR-0025) — `add index` / `drop index`,
   rebuild-preserving, persisted in `project.yaml`.
-  `NOT NULL`, `UNIQUE`, `CHECK`, `DEFAULT` still pending.)*
+  `NOT NULL` / `UNIQUE` / `CHECK` / `DEFAULT` done (ADR-0029) —
+  a constraint suffix on `create table` / `add column`, plus
+  `add constraint` / `drop constraint` on existing columns;
+  populated-column additions are guarded by a pre-flight
+  dry-run that refuses with a table of offending rows.)*
 - [~] **C3a** Modify relationship: `modify relationship <name>
   [on delete <action>] [on update <action>]`. Users can achieve
   the same via drop + add today; one-step modify is a small