feat(i18n): unit-mapped briefings with mechanical --apply, adopting the #684 planner mechanics

The briefing now maps each update at the narrowest safely aligned
granularity, widening deterministically on mapping failure: a change
confined to the pair's byte-identical code fences is computed outright
(--apply splices it into the counterpart and validates the result
against the pairing gate's structural signature before writing);
otherwise changed Markdown units — headings, paragraphs, table rows,
list items, fences, block quotes, HTML blocks, thematic breaks, link
definitions, matched by container-scoped kind sequences — each carry
their last-confirmed source, current source, and current counterpart
text; units that do not align fall back to depth-matched heading
sections (depth only, so translated heading text still maps); and when
sections do not align either, or both sides drifted, the briefing says
so and withholds the mapping. Terminology rows now match the changed
spans only, English terms on word boundaries with plural inflections,
and Chinese-target briefings track each relevant term's document-wide
first occurrence — a moved occurrence pulls the vacated and receiving
spans into the briefing with an explanatory note.

The unit mapping, mechanical code splice, and first-occurrence tracking
adopt the planner design from the incremental prompt-pipeline PR (#684),
whose provider-backed bake-off independently validated the same scope
ladder; this PR carries those mechanics into the agent-facing briefing
path so both consumers of the consistency records behave alike. The
prior line-hunk section mapping and its heading-text alignment (which
could not map cross-language sections) are replaced wholesale.

Docs: SKILL.md update path, i18n README pair, development.md pair, and
the briefed-updates Agent Note pair brought along; the development.md
fence edit was applied with --apply itself, and the prose updates were
made through the new unit/section briefings.

docs/i18n/README.zh.md

@@ -15,7 +15,7 @@
   foo.zh.md: 89e6c98d92887913cadf06b2adb97f26cde4849b
   ```
 
-  用 blob hash 而不是 commit hash，这样同一个 PR 里改动的文件也能算出记录（`git hash-object foo.md`），一致性是纯内容比较。记录的 hash 还能还原任一侧上次确认时的确切文本，所以失去同步的配对是「按被改一侧的 diff 最小化地修补另一侧」，从不整篇重译。`pnpm run gen-translation-brief <pair>` 会机械地汇集这次更新的工作集：被改一侧自上次确认以来的 diff、该 diff 落入的对侧文件小节、触及的术语表行，以及有约束力的更新规则（[briefed-updates Agent Note](../../.agents/notes/implemented/process/2026-07-26-briefed-minimal-translation-updates.md)）。两侧对齐后，`pnpm run verify-translation-pairing --write <pair>` 重新记录两个 hash；那份 yaml diff 就是「确认一致」这个动作本身，可以被评审，也正因如此，`--write` 要求点名你确认过的配对（`--write --all` 是显式的全语料形式）。
+  用 blob hash 而不是 commit hash，这样同一个 PR 里改动的文件也能算出记录（`git hash-object foo.md`），一致性是纯内容比较。记录的 hash 还能还原任一侧上次确认时的确切文本，所以失去同步的配对是「按被改一侧的 diff 最小化地修补另一侧」，从不整篇重译。`pnpm run gen-translation-brief <pair>` 会以能安全对齐的最窄粒度——先是有改动的 Markdown 单元，再是标题小节，最后是整篇文档——机械地汇集这次更新的工作集：被改一侧自上次确认以来的 diff、每个改动块的三方文本、改动触及的术语表行，以及有约束力的更新规则；仅落在配对中逐字节一致的围栏代码块内的改动可以直接算出，`--apply` 则经结构签名校验后把它拼接进对侧文件（[briefed-updates Agent Note](../../.agents/notes/implemented/process/2026-07-26-briefed-minimal-translation-updates.md)）。两侧对齐后，`pnpm run verify-translation-pairing --write <pair>` 重新记录两个 hash；那份 yaml diff 就是「确认一致」这个动作本身，可以被评审，也正因如此，`--write` 要求点名你确认过的配对（`--write --all` 是显式的全语料形式）。
 - **语言切换行。** 两个文件在各自 H1 标题之后立即互链：英文文件带 `English | [中文](foo.zh.md)`，中文文件带 `[English](foo.md) | 中文`。
 - **结构与另一侧一一对应。** 标题深度与顺序、列表类型、有序列表起始编号、列表项数量、表格行列数、链接目标与逐字节一致的代码块在配对两侧一一对应；完整保持规则见 [translation-rules.md](translation-rules.md)。既有 Markdown 门禁对 `.zh.md` 文件原样生效（`verify-md-wrap`、`verify-md-links`）。