docs: archive the finished backlogs (RD-30)

Two backlog trees are complete: `docs/project/backlog/` (75 files, every
WP done) and `docs/project/refactor-backlog-setup/` (the arc before it).
Move both under `docs/project/archive/` with `git mv`, so history stays
intact through `git log --follow`. `SHOWCASE-ROADMAP.md` moves with them,
because it points at the now-archived backlog README.

Add `docs/project/archive/README.md`. It states that these trees are
historical and names the two directories that are still live.

Repoint every inbound reference named in RD-30's Files table: CLAUDE.md,
the root README, both backend READMEs, `LetterHtml.cs`, `a11y.mdx`, the
`document-feature` and `new-ssp` skills, and the readable-codebase PLAN,
README, and RD-19 ticket. Fix two upward-relative links inside the moved
WP files (WP-68, WP-69) that gained a directory level and would otherwise
break. Repoint `.prettierignore`'s two agent-prompt exclusions to their
new path, so prettier keeps leaving those files' exact wording alone.

Mark RD-30 done and check off its acceptance criteria; flip its README
row to done.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/project/archive/backlog/WP-39-showcase-snippets-animations.md

@@ -0,0 +1,49 @@
+# WP-39 — Showcase: linked code snippets + teaching animations
+
+Status: done
+Phase: 8 — platform/DX/showcase
+Priority: P1
+
+## Outcome
+
+`?raw` imports are NOT supported by Angular's esbuild build (verified), so anti-drift is done via
+a generator (mirrors `gen:api`): `// #region showcase:<name>` markers in real source
+(registration.ts, remote-data.ts, postcode.ts, change-request.machine.ts, intake.machine.ts) →
+`scripts/gen-snippets.mjs` (`npm run gen:snippets`) → committed `src/app/showcase/snippets.generated.ts`.
+A CI drift gate (`gen:snippets && git diff --exit-code`) forces a regen after any edit, so the shown
+code IS the shipped code. The showcase renders the 5 real snippets (union/fold/parse/machine/steps)
+each with a `↳ <source file>` caption, highlighted by a tiny dependency-free `highlightTs` (+spec);
+the deliberately-wrong illustrations stay authored (no real source to link). Teaching animation: the
+active state node pops (scale) as the wizard transitions, and the parse result animates in
+(`animate.enter`), reusing the existing reduced-motion-safe toolkit. Generated file is
+prettier-ignored (like api-client.ts) so the drift gate and format:check don't conflict.
+
+## Why
+
+The `/concepts` showcase shows live components, but its code snippets are hand-authored
+`innerHTML` strings with manual highlighting — they drift from the real code. And the animation
+toolkit (`withViewTransitions`, native `animate.enter/leave`, reduced-motion-safe keyframes in
+`styles.scss`) is proven in dashboard/upload but unused for teaching motion.
+
+## Decisions
+
+- **Anti-drift snippets:** load _real_ source excerpts via Vite `?raw` imports with `// #region name`
+  markers + a tiny pure extractor; render with lightweight highlighting. No more `innerHTML` literals.
+- On the Storybook/MDX side, use `<Source>`/Canvas "show code" consistently (extend beyond the
+  current 4 of 14 pages).
+- **Reuse** the existing motion toolkit (no new animation lib): `animate.enter/leave` on the
+  fold/union demos + a "step through `reduce`" animation for the TEA loop; keep reduced-motion-safe.
+
+## Files
+
+- `src/app/showcase/concepts.page.ts` — replace `innerHTML` snippets with `?raw`+region excerpts;
+  apply animations.
+- New tiny extractor util (pure) + spec (region slicing).
+- `src/docs/machines.mdx`, `fp-in-ui.mdx`, `remote-data.mdx` — reference the live linked snippets.
+
+## Acceptance criteria
+
+- [x] The good-side snippets are sourced from real files via region markers (edit source → regen →
+      snippet updates); a CI drift gate enforces it. Bad-pattern illustrations stay authored.
+- [x] The state machine + parse demos animate (reduced-motion respected globally); no new dependency.
+- [x] `npm run ci` green.