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/development.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 docs/development.md
-development.md: 4aab6772a514c5c461535f6e37906a503c10215a
-development.zh.md: c420c0132d5d945457bad73ae71691d345488150
+development.md: fd7f39ae7b5aac2d44572979ca8c8f1d2df0de6f
+development.zh.md: 7dd6209bad75d605e0056d2465a35b08aa091780