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.

docs/i18n/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write
-README.md: 6e2bbd27c3288037bafeb6cc71b801d56b956ab4
-README.zh.md: 04c99ae336cf1e96cbc185f0ccbd063ef8977944
+README.md: ba3431c72d9f2a86220df4ed682620514401291f
+README.zh.md: 02957c97937525a1da27242620a8327b863de040