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.

.agents/skills/dsh-translate-docs/SKILL.md

@@ -19,9 +19,9 @@ description: Use when creating or updating the bilingual counterpart of a doc in
 
 Benchmarked on real pair updates from this repo's history, the briefing-driven path costs a fraction of a guidance-corpus-loading run at equal measured quality; the [briefed-updates Agent Note](../../notes/implemented/process/2026-07-26-briefed-minimal-translation-updates.md) holds the evidence.
 
-1. **Generate the briefing**: `pnpm run gen-translation-brief <any file of the pair>` (no arguments briefs every out-of-sync pair). The briefing contains the authored side's diff since the last confirmed-consistent state, the counterpart sections that diff lands in (with current line numbers), the terminology rows the diff touches, and a digest of the binding update rules.
-2. **Mechanical-only diff? Apply it directly.** If every changed line lies inside code fences that the pair shares byte-identically, the counterpart edit is byte-copying with no translation judgment; the orchestrator applies it without spawning a subagent.
-3. **Prose diff? Delegate to a subagent, passing the briefing** (or the command to generate it). The briefing is the translator's whole working set — the subagent does not re-read the guidance corpus (the rules digest and terminology rows are inline, and the counterpart's own surrounding text carries the established renderings) and does not re-derive the diff. It escalates to the whole-document path's sources of truth only when the briefing leaves a specific decision genuinely unanswerable — an unlisted term with no precedent in the surrounding text, or a `BOTH sides changed` warning, which always means reconciling by hand under [translation-rules.md](../../../docs/i18n/translation-rules.md).
+1. **Generate the briefing**: `pnpm run gen-translation-brief <any file of the pair>` (no arguments briefs every out-of-sync pair). The briefing maps the change at the narrowest safely aligned granularity — changed Markdown units (paragraph, table row, list item, heading), then whole heading sections, then whole document — and contains the authored side's diff since the last confirmed-consistent state, each changed unit's last-confirmed source, current source, and current counterpart text (with line numbers), the terminology rows the change touches, first-occurrence movement notes, and a digest of the binding update rules.
+2. **Mechanical-only diff? `--apply` it.** When every change lies inside code fences that the pair shares byte-identically, the briefing says so; `pnpm run gen-translation-brief --apply <pair>` splices the edited fences into the counterpart and structure-validates the result before writing — no subagent, no hand-editing.
+3. **Prose diff? Delegate to a subagent, passing the briefing** (or the command to generate it). The briefing is the translator's whole working set — the subagent does not re-read the guidance corpus (the rules digest, terminology rows, and each changed unit's three-way context are inline) and does not re-derive the diff. It escalates to the whole-document path's sources of truth only when the briefing leaves a specific decision genuinely unanswerable — an unlisted term with no precedent in the surrounding text, or a whole-document briefing (`BOTH sides changed`, or neither units nor sections align), which always means reconciling by hand under [translation-rules.md](../../../docs/i18n/translation-rules.md).
 4. **Smallest edit that covers the diff.** Preserve the reviewed phrasing of everything the diff does not touch, then verify the changed hunks clause by clause against the source: nothing added, nothing dropped, terminology per the inline rows, code spans verbatim.
 5. **Record and verify, scoped**: `pnpm run verify-translation-pairing --write <pair>` then `pnpm run verify-translation-pairing <pair>`. `--write` names exactly the pairs you confirmed — it refuses to run bare so a bulk re-record is always an explicit `--all`. The corpus-wide check still runs in `doc-sync`/CI; do not run it per-update.