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",

scripts/verify-translation-pairing.ts

@@ -21,7 +21,10 @@
  *      depths in order, fenced code blocks VERBATIM (info string + content),
  *      table column counts, list kinds, and every link target except the
  *      switcher itself.
- *   3. `excluded` files (generated docs, agent instructions, the bilingual
+ *   3. Date-named documents (`yyyy-mm-dd-*.md`, i.e. RFCs) dated on/after the
+ *      manifest's `requiredSince` merge bilingual — the frontier for NEW
+ *      documents, independent of the `required` back-catalog list.
+ *   4. `excluded` files (generated docs, agent instructions, the bilingual
  *      terminology table) have no `.zh.md` and no `.i18n.yaml` at all.
  *
  * What it deliberately does NOT check is translation quality or which side
@@ -63,6 +66,8 @@ const SCOPE_PATTERNS = ['README.md', 'README.zh.md', 'README.i18n.yaml', 'docs/*
 interface Manifest {
   required: string[]
   excluded: string[]
+  /** Date-named documents (yyyy-mm-dd-*.md, i.e. RFCs) dated on/after this day must merge bilingual. */
+  requiredSince: string
 }
 const manifest = JSON.parse(readFileSync(join(root, 'scripts/translation-pairing.manifest.json'), 'utf8')) as Manifest
 
@@ -250,7 +255,22 @@ for (const req of manifest.required) {
   }
 }
 
-// 2. Every pair that exists at all is complete and consistent. Anchor on the
+// 2. Date-named documents (RFCs) dated on/after the requiredSince cutoff merge
+// bilingual: a new RFC lands with its pair or not at all. Deterministic from
+// the filename alone — no git history, so it holds on shallow CI checkouts.
+const DATED = /(\d{4}-\d{2}-\d{2})-[^/]*\.md$/
+for (const source of sources) {
+  if (isExcluded(source)) continue
+  const dated = DATED.exec(source)
+  if (!dated?.[1] || dated[1] < manifest.requiredSince) continue
+  const { zh } = pairPaths(source)
+  if (!existsSync(join(root, zh))) {
+    errors.push(`${source}: dated ${dated[1]} — documents dated on/after ${manifest.requiredSince} merge bilingual (docs/i18n/README.md); add the counterpart and record the pair`)
+    state.set(source, 'missing')
+  }
+}
+
+// 3. Every pair that exists at all is complete and consistent. Anchor on the
 // union of .zh.md files and .i18n.yaml records so a half-deleted pair is
 // caught from either remnant.
 const pairAnchors = new Set<string>()
@@ -317,7 +337,9 @@ if (listMode) {
   const rows = [...state.entries()].sort((a, b) => order[a[1]] - order[b[1]] || a[0].localeCompare(b[0]))
   for (const [file, status] of rows) {
     const required = manifest.required.includes(file)
-    console.log(`${status.padEnd(11)} ${file}${status === 'missing' ? (required ? '  (required)' : '  (backlog)') : ''}`)
+    const date = DATED.exec(file)?.[1]
+    const tag = required ? '  (required)' : date && date >= manifest.requiredSince ? '  (required by date)' : '  (backlog)'
+    console.log(`${status.padEnd(11)} ${file}${status === 'missing' ? tag : ''}`)
   }
   const counts = { 'ok': 0, 'out-of-sync': 0, 'missing': 0 }
   for (const status of state.values()) counts[status]++