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/README.md

@@ -124,7 +124,7 @@ two. Note that RD-15 exists because 22 abandoned agent worktrees are still on di
 | RD-27 | **The layer move:** 33 `git mv` + 28 specifiers + 8 MDX imports              | 21         | yes       | done   |
 | RD-28 | Layer-tag fixes + the `libs/beheer` title rule                               | 27         | yes       | done   |
 | RD-29 | The 3 atomic-ladder rules in dependency-cruiser                              | 27         |           | done   |
-| RD-30 | Archive the finished backlogs (16,300 lines) + an archive README             | 01         |           | todo   |
+| RD-30 | Archive the finished backlogs (16,300 lines) + an archive README             | 01         |           | done   |
 | RD-31 | `ARCHITECTURE.md` section 6a: symbols not lines, 2 dead paths, new names     | 03, 08, 16 |           | todo   |
 | RD-32 | `fp-tea-atomic-design.md`: 11 broken paths + the broken anchor               | 27         |           | todo   |
 | RD-33 | CLAUDE.md + `atomic-design.mdx` + the `ui-component` skill                   | 03, 27, 29 | yes       | todo   |
@@ -145,7 +145,7 @@ four import lines, and it collides with nothing else in the table — RD-27's mo
 `libs/shared/src/ui/`. Pull it forward into any short session. It is numbered last only because
 it was added after RD-04 shipped.
 
-**RD-37 must run before RD-30**, despite its number. RD-30 archives `docs/project/backlog/`,
+**RD-37 must run before RD-30**, despite its number. RD-30 archives `docs/project/archive/backlog/`,
 and five of the paths it would have to rewrite point at `WP-11-markup-fidelity.md` from
 accessibility suppressions that RD-37 either deletes or re-aims. Doing RD-30 first means
 rewriting five paths that are about to change again — and enshrining a promise nobody owns.