docs,feat(doc-gates): fix 15 dead anchor fragments; verify-md-links now validates fragments

A corpus sweep under the doc/prose standards found 15 links whose #fragment
named no anchor in its target — reworded headings, one relocated contract
(tool-fs → the group README's no-timeout rule), and zh sides citing English
slugs their Chinese headings never produce. Fixed all 15 (zh sides get the
conventional explicit <a id> + English fragment), fixed the one generator-owned
instance at its source (gen-doc-graphs), and extended verify-md-links to
resolve fragments onto Markdown targets — same-file anchors included — against
heading slugs and explicit <a id>, so the class is gated instead of manually
grepped. Remaining probes (narrated history, duplication shingles, comment
transcripts, budgets) came back clean; sibling-adapter README symmetry and
implemented-note contrasts are deliberate keeps.

scripts/verify-md-links.spec.ts

@@ -0,0 +1,83 @@
+/**
+ * Acceptance-path coverage for fragment validation in `verify-md-links`: a
+ * `#fragment` onto a Markdown target — same-file anchors included — must name
+ * a real heading slug or explicit `<a id>`, while non-Markdown fragments and
+ * external targets stay out of scope.
+ */
+
+import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, describe, expect, it } from 'vitest'
+import { documentAnchors, findViolations, githubSlug } from './verify-md-links.ts'
+
+const roots: string[] = []
+afterEach(() => {
+  for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true })
+})
+
+function layout(files: Record<string, string>): string {
+  const root = mkdtempSync(join(tmpdir(), 'md-links-'))
+  roots.push(root)
+  for (const [rel, content] of Object.entries(files)) {
+    mkdirSync(join(root, rel, '..'), { recursive: true })
+    writeFileSync(join(root, rel), content)
+  }
+  return root
+}
+
+function violationsIn(root: string, rel: string): { url: string; reason: string }[] {
+  const cache = new Map<string, Set<string>>()
+  const anchorsOf = (abs: string): Set<string> => {
+    const hit = cache.get(abs)
+    if (hit) return hit
+    const anchors = documentAnchors(readFileSync(abs, 'utf8'))
+    cache.set(abs, anchors)
+    return anchors
+  }
+  return findViolations(join(root, rel), anchorsOf, root).map(({ url, reason }) => ({ url, reason }))
+}
+
+describe('documentAnchors', () => {
+  it('slugs headings, suffixes repeats, and reads explicit <a id> anchors', () => {
+    const anchors = documentAnchors([
+      '# My Doc',
+      '## Live `events` — mode!',
+      '## Repeat',
+      '## Repeat',
+      '<a id="hand-anchor"></a>',
+      '',
+    ].join('\n'))
+    expect(anchors).toEqual(new Set(['my-doc', 'live-events--mode', 'repeat', 'repeat-1', 'hand-anchor']))
+    expect(githubSlug('Security and authority are non-goals')).toBe('security-and-authority-are-non-goals')
+  })
+})
+
+describe('findViolations fragments', () => {
+  it('accepts resolving same-file and cross-file fragments, non-md fragments, and externals', () => {
+    const root = layout({
+      'a.md': '# A\n\n## Deferred work\n\n[self](#deferred-work) [b](b.md#part-two) [code](x.ts#L10) [ext](https://x.example/#frag)\n',
+      'b.md': '# B\n\n## Part two\n',
+      'x.ts': 'export {}\n',
+    })
+    expect(violationsIn(root, 'a.md')).toEqual([])
+  })
+
+  it('rejects a same-file fragment that names no heading or <a id>', () => {
+    const root = layout({ 'a.md': '# A\n\n[gone](#deferred-work)\n' })
+    expect(violationsIn(root, 'a.md')).toEqual([{ url: '#deferred-work', reason: 'anchor' }])
+  })
+
+  it('rejects a cross-file fragment missing from the target document', () => {
+    const root = layout({
+      'a.md': '# A\n\n[stale](b.md#old-heading)\n',
+      'b.md': '# B\n\n## New heading\n',
+    })
+    expect(violationsIn(root, 'a.md')).toEqual([{ url: 'b.md#old-heading', reason: 'anchor' }])
+  })
+
+  it('still rejects a missing target file, reported as target not anchor', () => {
+    const root = layout({ 'a.md': '# A\n\n[ghost](missing.md#anything)\n' })
+    expect(violationsIn(root, 'a.md')).toEqual([{ url: 'missing.md#anything', reason: 'target' }])
+  })
+})