feat(scripts): generate the RFC index tables from the tree

docs/rfc/README.md's per-lifecycle tables are now generated between
gen-rfc-index marker comments from each RFC's path (lifecycle/class),
H1 title (optional 'RFC: ' prefix stripped), and filename date, sorted
by date then filename — the one docs region every proposal wave edits
and every concurrent branch conflicts on becomes derived state.

scripts/rfc-index.ts owns the shared walker (closed lifecycle/class
sets, structure rules, parseable-H1 requirement) and the renderer;
gen-rfc-index.ts is the writer CLI; verify-rfc-classification.ts keeps
the structure check and asserts the committed regions byte-match a
fresh render (freshness subsumes the index-completeness check, since a
generated-from-disk table is definitionally complete and correctly
headed). A malformed H1 is a hard error in both directions, so the H1
is now load-bearing as the title source — the one nonconforming H1
(a status suffix duplicating the path) is normalized.

Implements docs/rfc/implemented/process/2026-07-04-generate-rfc-index-tables.md
(moved from proposed/ and amended to the shipped mechanics); the
classification RFC's verify-only stance carries the supersession
cross-link per implemented/AGENTS.md.

scripts/gen-rfc-index.ts

@@ -0,0 +1,30 @@
+/**
+ * Regenerate the RFC index tables in `docs/rfc/README.md` from the RFC tree
+ * (see [rfc-index.ts](./rfc-index.ts) for the layout contract and rendering
+ * rules). Rewrites ONLY the marker-delimited regions; the curated prose is
+ * untouched. Freshness is asserted by `verify-rfc-classification.ts` (a
+ * `doc-sync` member), so a stale committed index fails CI.
+ *
+ * Run: `pnpm run gen-rfc-index`.
+ */
+
+import { readFileSync, writeFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+import { rfcRoot, spliceReadme, walkRfcTree } from './rfc-index.ts'
+
+const { rfcs, errors } = walkRfcTree()
+if (errors.length > 0) {
+  console.error('gen-rfc-index: refusing to generate from a structurally invalid tree:')
+  for (const e of errors) console.error(`  ${e}`)
+  process.exit(1)
+}
+
+const readmePath = resolve(rfcRoot, 'README.md')
+const readme = readFileSync(readmePath, 'utf8')
+const next = spliceReadme(readme, rfcs)
+if (next === readme) {
+  console.log(`gen-rfc-index: docs/rfc/README.md is up to date (${rfcs.length} RFCs).`)
+} else {
+  writeFileSync(readmePath, next)
+  console.log(`gen-rfc-index: docs/rfc/README.md regenerated (${rfcs.length} RFCs).`)
+}