docs: mark H1 done — friendly DB-error layer is shipped

Verification found H1 (ADR-0019) fully implemented and tested: the
friendly::translate_error chokepoint is wired on the live failure path
(runtime + app + DbError::friendly_message), covers all five error
categories (UNIQUE, FOREIGN KEY both sides, NOT NULL, CHECK,
type-mismatch) with operation×kind×verbosity wording, the messages
verbosity command, and §6 row-pinpointing via runtime-resolved facts —
backed by 44 friendly unit tests + 12 full-stack friendly_enrichment
integration tests. The "partial / FK-only" notes were stale.

Mark requirements H1 done; fix the obsolete "diagnostic_table is
always None" comment in translate.rs (pinpointing landed in 431645a).
Remaining ADR-0019 scope (§9 i18n sweep, §OOS-2 advanced-SQL
sanitization, §OOS-3 messages persistence) stays deferred.

docs/requirements.md

@@ -542,12 +542,21 @@ since ADR-0027.)
 
 ## Hints, help, errors
 
-- [ ] **H1** Friendly error-rewriting layer translates SQLite
-  error messages into learner-friendly equivalents.
-  *(Progress: foreign-key constraint failures are enriched
-  with both inbound and outbound relationship listings (so
-  RESTRICT errors point at the children that still reference
-  this table); full SQL → English translation pending.)*
+- [x] **H1** Friendly error-rewriting layer translates engine
+  error messages into learner-friendly equivalents (ADR-0019).
+  *(Done: the `friendly::translate_error` chokepoint is wired on
+  the live failure path (runtime + app + `DbError::friendly_message`)
+  and covers all five ADR-0019 §3 categories — UNIQUE, FOREIGN KEY
+  (parent- and child-side), NOT NULL, CHECK, and type-mismatch —
+  with operation×kind×verbosity catalog wording, the
+  `messages short|verbose` verbosity command, and §6 row-pinpointing
+  via runtime-resolved facts rendered through the bordered
+  diagnostic table. Covered by 44 `friendly` unit tests + 12
+  full-stack `friendly_enrichment` integration tests. Remaining
+  ADR-0019 scope is deferred and separately tracked: the §9 i18n
+  migration sweep of all other user-facing strings, advanced-mode
+  SQL-error sanitization (§OOS-2), and `messages` persistence
+  (§OOS-3, awaits the settings ADR).)*
 - [ ] **H1a** Strong syntax-help in parse errors. When the user
   types something near-correct (e.g. `insert into T ('Oli')` —
   forgotten `values`; or `update T set x=1` — missing WHERE),