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/readable-codebase/PLAN.md

@@ -49,7 +49,7 @@ ticket converts it into the artifact this repo already uses for exactly this.
 
 ### Reuse the existing protocol, do not invent one
 
-`docs/project/backlog/README.md` is a proven mechanism — 75 work packages driven to `done`
+`docs/project/archive/backlog/README.md` is a proven mechanism — 75 work packages driven to `done`
 through it, and its own notes say the tickets are _"self-contained (each WP file carries its
 own current-state handoff) and sized for a fresh Sonnet session."_ Copy it wholesale:
 
@@ -61,7 +61,7 @@ own current-state handoff) and sized for a fresh Sonnet session."_ Copy it whole
 - **A runnable GREEN one-liner** as the global definition of done.
 
 New home: **`docs/project/readable-codebase/`**, prefix **`RD-NN`**. It must not extend
-`docs/project/backlog/`, because Phase 5 archives that directory — a finished arc gets
+`docs/project/archive/backlog/`, because Phase 5 archives that directory — a finished arc gets
 archived, a new arc gets its own folder. `RD-` also avoids collision with the existing
 `WP-`/`RB-` prefixes, which matters because Phase 2 greps for those.