docs(fase-20): history doc + CHANGELOG [Unreleased] entry

History doc covers: ENUM-vs-CHECK rationale (plan was written against
the wrong primitive — actual gate is the language_code ENUM from
migration 001), machine-translation seed + [needs review] marker,
ICU-placeholder parity verification, locale-keys diff script
contract, and the ParaglideJS {#key lang}-on-navigation cross-cutting
gap that constrained the shape of EU-01.

Test deltas: pgTAP +5, integration +2, unit +4, e2e +2 — 581 + 3
skipped (was 568 + 3). Documents the pre-existing 010_sync_conflicts
flake (offline.test.ts e2e contamination) as out of scope for Fase 20.

CHANGELOG gains the [Unreleased] entry. Moves into [v1.0.0] in the
release commit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CHANGELOG.md

@@ -11,6 +11,7 @@ labels for the area, not log entries.
 
 - [new] Required list title + auto-numbered prefix + admin-curated title catalog (Fase 18).
 - [new] Emoji picker expanded with full food + animals categories (Fase 19).
+- [new] Euskera (Basque) locale (Fase 20). Machine-translated seed pending native review.
 
 ## [v0.0.0-beta] — 2026-05-19 — MVP + MVP2 (Fases 0–16)