feat(i18n): briefing generator and pair-scoped pairing gate

gen-translation-brief assembles the minimal-update working set for an
out-of-sync pair from its consistency record: the authored side's diff
since last confirmation, the counterpart sections that diff lands in
(heading-mapped only where the last-confirmed structures align), the
terminology rows the diff touches, and a per-direction rules digest.

verify-translation-pairing now accepts pair paths to check just the
named pairs during update iteration; --write requires naming the
confirmed pairs (--write --all is the explicit corpus form) so a bulk
re-record can no longer silently bless drifted pairs the caller never
reviewed. Each record's comment names its own scoped command.

scripts/translation-pairing.spec.ts

@@ -3,7 +3,9 @@
 import { describe, expect, it } from 'vitest'
 import {
   isTranslationScopeFile,
+  pairAnchorOfArgument,
   parseTranslationMarkdown,
+  parseTranslationPairingCliArgs,
   parseTranslationPairingManifest,
   translationStructureDiff,
   translationStructureSignature,
@@ -102,3 +104,40 @@ describe('translation structural signature', () => {
     ])
   })
 })
+
+describe('pair CLI arguments', () => {
+  it('normalizes any pair file or bare stem to the English anchor', () => {
+    expect(pairAnchorOfArgument('docs/foo.md')).toBe('docs/foo.md')
+    expect(pairAnchorOfArgument('docs/foo.zh.md')).toBe('docs/foo.md')
+    expect(pairAnchorOfArgument('docs/foo.i18n.yaml')).toBe('docs/foo.md')
+    expect(pairAnchorOfArgument('docs/foo')).toBe('docs/foo.md')
+    expect(pairAnchorOfArgument('.\\docs\\foo.zh.md')).toBe('docs/foo.md')
+  })
+
+  it('scopes a check to named pairs and dedupes the three spellings', () => {
+    expect(parseTranslationPairingCliArgs(['docs/foo.zh.md', 'docs/foo.i18n.yaml', 'docs/bar.md'])).toEqual({
+      mode: 'check',
+      scope: 'pairs',
+      anchors: ['docs/bar.md', 'docs/foo.md'],
+    })
+    expect(parseTranslationPairingCliArgs([])).toEqual({ mode: 'check', scope: 'corpus', anchors: [] })
+  })
+
+  it('requires --write to name confirmed pairs or opt into --all', () => {
+    expect(() => parseTranslationPairingCliArgs(['--write'])).toThrow('requires the pair(s) you confirmed')
+    expect(parseTranslationPairingCliArgs(['--write', 'docs/foo.md'])).toEqual({
+      mode: 'write',
+      scope: 'pairs',
+      anchors: ['docs/foo.md'],
+    })
+    expect(parseTranslationPairingCliArgs(['--write', '--all'])).toEqual({ mode: 'write', scope: 'corpus', anchors: [] })
+    expect(() => parseTranslationPairingCliArgs(['--write', '--all', 'docs/foo.md'])).toThrow('not both')
+  })
+
+  it('keeps --list corpus-only and rejects unknown flags', () => {
+    expect(parseTranslationPairingCliArgs(['--list'])).toEqual({ mode: 'list', scope: 'corpus', anchors: [] })
+    expect(() => parseTranslationPairingCliArgs(['--list', 'docs/foo.md'])).toThrow('takes no other flags or paths')
+    expect(() => parseTranslationPairingCliArgs(['--all'])).toThrow('--all only applies to --write')
+    expect(() => parseTranslationPairingCliArgs(['--frobnicate'])).toThrow('unknown flag(s): --frobnicate')
+  })
+})