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-27-brief-ux-layer.md

@@ -0,0 +1,97 @@
+# WP-27 — Brief UX layer (undo/redo, standaardbrief, search, diff badges)
+
+Status: done
+Phase: 6 — Brief v2 (edit-on-the-letter, org templates, server-rendered preview)
+
+## Why
+
+PRD Brief v2 §7: the working-day features that make the composer pleasant daily.
+Several are nearly free **because** state is one immutable value — that's the
+teaching payload: undo/redo is a shell-side snapshot list, the rejection diff is a
+pure function over two values. Say so in code comments and stories.
+
+## Read first
+
+- PRD Brief v2 §7 (and the plan-review trim recorded below)
+- `src/app/brief/application/brief.store.ts` (autosave + `SaveState` already exist)
+- `src/app/brief/domain/brief.machine.ts` — the `Seed` Msg (undo/redo's restore path)
+
+## Decisions (pre-made, don't relitigate)
+
+- **Trim agreed at plan review.** IN: undo/redo, autosave retry affordance,
+  standaardbrief, passage search, canvas zoom controls, Ctrl+Z/Ctrl+Shift+Z,
+  block-level rejection-diff badges. OUT (deferred, one line each in Out of scope):
+  soft lock/takeover, case-context panel, 401 autosave grace, per-user usage counts,
+  shortcut-overlay dialog, inline character-level text diff.
+- **Undo/redo is shell state, not machine state**: a `past`/`future: Brief[]` stack in
+  `BriefStore` (cap 50; push on `edit()`; clear `future` on a new edit); restore
+  dispatches the **existing `Seed` Msg** — zero machine changes — then `scheduleSave()`.
+  (Superseded by WP-31: the hand-rolled stack was later extracted into the shared
+  `createHistory<T>(cap)` helper — `BriefStore` now holds `this.history =
+createHistory<Brief>(50)` and calls `.record`/`.undo`/`.redo`/`.clear` on it; same
+  cap-50, push-on-edit, clear-on-new-edit semantics, just generalized for reuse.)
+- **Standaardbrief**: backend seeds `IsDefault` on 2–3 kern passages
+  (`LibraryPassageDto` gains the flag); one button, visible only while the kern
+  section is empty, dispatches the existing `PassagesInserted` with the default set —
+  one Msg, one undo step.
+- **Passage search is a client-side filter** in the picker (label + content match) —
+  the library is small; no server search, no usage tracking.
+- **Rejection diff**: pure `diffBlocks(before, after): BlockDiff[]` in
+  `domain/brief-diff.ts` (added/removed/changed by `blockId`); the "before" snapshot
+  is captured shell-side when the `Rejected` dispatch happens (POC limit: lost on
+  reload — comment it). Rendered as "gewijzigd sinds afwijzing" badges on the canvas;
+  the approver gets a "Toon wijzigingen" toggle on resubmission.
+- **Autosave retry**: `SaveState.Error` already exists; add the "Opnieuw proberen"
+  button that calls the existing flush path. No new state.
+
+## Files
+
+- `src/app/brief/application/brief.store.ts` (+spec: history bounds, clear-on-edit,
+  redo, rejection snapshot)
+- `src/app/brief/domain/brief-diff.ts` (new, +spec)
+- `backend/src/BigRegister.Api/Data/BriefStore.cs` (`IsDefault` seed) +
+  `Contracts/Dtos.cs` (`LibraryPassageDto`) + gen:api + adapter parse
+- `src/app/brief/ui/passage-picker/*` (search input)
+- `src/app/brief/ui/letter-canvas/*` (diff badges, standaardbrief button, zoom controls)
+- `src/app/brief/ui/brief.page.ts` (undo/redo buttons + keydown listener, retry button)
+
+## Steps
+
+1. `diffBlocks` + spec (added/removed/changed/unchanged; changed = same blockId,
+   different content).
+2. Store: history + undo/redo + rejection snapshot (+spec).
+3. Backend `IsDefault` + gen:api + parse.
+4. UI: standaardbrief button, search, zoom, badges, keyboard, retry.
+5. Stories for the new states (axe).
+
+## Acceptance criteria
+
+- [x] Remove a block → Ctrl+Z restores it → Ctrl+Shift+Z re-removes; buttons mirror;
+      history capped at 50; a new edit clears redo; restore re-triggers autosave.
+- [x] Empty kern + "Standaardbrief invoegen" → default passages inserted as one undo
+      step; button gone once kern is non-empty.
+- [x] Search filters passages by label and content.
+- [x] Reject → edit → resubmit: approver toggles "Toon wijzigingen", changed/added/
+      removed blocks are badged (block granularity).
+- [x] Autosave failure shows "Niet opgeslagen — opnieuw proberen"; retry works;
+      content never lost locally.
+- [x] Full GREEN.
+
+## Verification
+
+GREEN one-liner; store + diff specs; manual reject→edit→diff walk with two roles.
+
+## Out of scope (deferred, per plan review)
+
+Soft lock/heartbeat/takeover (real session infra, no FP teaching value here).
+Case-context panel (no case data exists). 401 autosave grace (auth is faked).
+Per-user passage usage counts (bookkeeping, demos nothing). Shortcut overlay dialog
+(no modal component exists; not worth building one). Inline character-level diff
+(block granularity carries the teaching point).
+
+## Risks
+
+Undo history holds `Brief` snapshots — deep-frozen immutable values, so sharing is
+safe, but never push non-content dispatches (status transitions, `Seed` itself) into
+history or undo will replay workflow state. The rejection snapshot lives in memory
+only — document it where it's captured.