docs(i18n): address ds-review-bot round-2 — switcher placeholders, regex anchor, contract completeness

新增 {{source_filename}}/{{source_filename_zh}} 占位符：切换行指令
可执行化，新配对（源文件尚无切换行）也必须生成（critical）。加粗
后空格规则限定于拉丁字母/数字，不再与全角标点规则冲突。日期匹配
锚定 basename 开头，release-notes-2026-* 类文件不再误判。金标
development.zh 补回 RFC 指引句与 agent 首现括注、pre-push 描述改
为 check:pre-push 调度器实况。术语表补回单文件可执行 RFC 配对在用
的 5 行（VFS/deploy root/peer dependency/serving surface/wheel）。
双语 README 的门禁清单补第 4 条（日期分界强制配对）。

docs/i18n/README.md

@@ -26,6 +26,7 @@ This repo's documentation is read by people and agents both inside and outside t
 1. Every file listed as `required` in [scripts/translation-pairing.manifest.json](../../scripts/translation-pairing.manifest.json) has a complete pair.
 2. Every pair that exists at all — required or not — is complete and consistent: all three files present, each side's current blob hash equals the recorded one (editing either side without re-confirming the pair goes red), both sides carry the language switcher, and the structural signatures match in order — heading depths, verbatim code blocks (info string and content), table column counts, list kinds, and every link target apart from the switcher.
 3. Files listed as `excluded` have no `.zh.md` and no `.i18n.yaml` at all.
+4. Every date-named document (`yyyy-mm-dd-*.md`) dated on or after the manifest's `requiredSince` cutoff has a complete pair — new documents merge bilingual from birth.
 
 `pnpm run verify-translation-pairing --list` prints the current pairing state of every document in scope — missing, out-of-sync, or ok — and is the work list for translation batches. It never fails; it reports.