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-37-dev-switcher-reset.md

@@ -0,0 +1,31 @@
+# WP-37 — Dev-switcher reset fix (scenario/role)
+
+Status: done
+Phase: 8 — platform/DX/showcase
+
+## Why
+
+The WP-33 dev switcher can't reset scenario/role back to `default`/`drafter` — it gets "stuck"
+(e.g. on `slow`). Cause: `currentScenario()`/`currentRole()` read the URL `?scenario=`/`?role=`
+param **before** sessionStorage, so once a param is in the address bar, `location.reload()` (same
+URL) re-reads the stale value and overrides what the switcher just stored.
+
+## Decisions
+
+- Once the switcher is used, **sessionStorage is authoritative**; a leftover URL param must not
+  win. Strip both dev params from the URL (`history.replaceState`) before reloading.
+- Extract the URL rewrite as a **pure** `stripDevParams(href)` so it's unit-testable without
+  touching `location.reload()`.
+
+## Files
+
+- `src/app/shared/infrastructure/dev-params.ts` (+ `dev-params.spec.ts`) — pure `stripDevParams`.
+- `src/app/shared/ui/debug-state/debug-state.component.ts` — `switchRole`/`switchScenario` call
+  `applyAndReload()` (replaceState with stripped URL, then reload).
+
+## Acceptance criteria
+
+- [x] Switching scenario/role to any value (incl. default/drafter) sticks after reload, even when
+      a `?scenario=`/`?role=` param was in the URL.
+- [x] `stripDevParams` removes both params, keeps other params + path/hash (spec).
+- [x] `npm run ci` green.