feat(i18n): new documents merge bilingual — the requiredSince date frontier

New docs must not enlarge the translation backlog: any date-named
document (yyyy-mm-dd-*.md, i.e. an RFC) dated on/after the manifest's
requiredSince cutoff must merge with its complete pair, independent of
the required back-catalog list. Cutoff ships at 2026-07-05, so the
existing 2026-07-04 RFC wave is grandfathered into the batch backlog
and everything after is bilingual from birth.

The date comes from the filename, deliberately: it is deterministic
from tree content alone — no git history (shallow CI checkouts hold),
no PR base ref — matching the gate's pure-content design. A proposal
dated before the cutoff that merges later escapes the rule; that is
the graceful grandfathering of in-flight work, not a hole.

--list marks such files (required by date); contract and RFC updated
in both languages and the pairs re-recorded.

scripts/translation-pairing.manifest.json

@@ -1,4 +1,5 @@
 {
+  "requiredSince": "2026-07-05",
   "required": [
     "README.md",
     "docs/development.md",