docs(router): archive 18 stale/superseded docs - #551

What

Moves 18 stale docs — completed action-plans, applied handoffs, superseded research, and shipped RFCs — out of the live docs into an archive/ folder, so the maintained doc set only contains things that describe current, shipped reality. Docs-only, [skip ci].

This is the same instinct behind the matching-doc reframe: point-in-time process scaffolding rots as "current" docs. The flagging came from the classification pass (47 docs reviewed; these 18 were marked archive-candidates with evidence — self-labeled "Status: Implemented/Superseded", shipped PRs, old rsc-router package names).

Where things went

Two archive/ folders, one per doc tree (the docs span both):

• docs/archive/ ← 8 root docs: MIDDLEWARE_SPEC, design/cf-cache-store, the three research/build-chunking-*, rfc-use-fetch-loader, rsc-caching-design, trailing-slash-research
• packages/rangojs-router/docs/internal/archive/ ← 10 internal docs: the *-handoff, *-review-actions, *-action-plan, *-remediation-plan, test-quality-*, stability-next-steps-plan

Note on destination: you said "move to docs/internal/archive/". Since the set spans the root docs/ tree and the package internal/ tree, I used a per-tree archive/ rather than relocating root docs into the package. Say the word if you'd rather consolidate all 18 into one folder.

How it's safe

• git mv preserves history (git tracked all 18 as renames, ~98–99% similarity even with the added banner).
• Each archived doc gets a one-line > **Archived.** banner at the top — with a "superseded by / shipped in #N" pointer where one applies (e.g. clientChunks → commit 2a5eb679; prerender-passthrough → Passthrough() wrapper; rsc-caching-design → docs/design/caching.md).
• References updated: the two live docs that pointed into the set — README.md (3 nav links) and stability-roadmap.md (2 refs) — now point at the archive/ paths and are marked (archived). README gained a one-line pointer to internal/archive/.
• Cross-links within the archive set (the build-chunking trio; stability-next-steps-plan ↔ the review/plan docs) were same-tree, so a same-folder move keeps them resolving.
• Verified: zero remaining live links to a non-archive path of any archived doc. The cf-cache-store "references" were the source file cf-cache-store.ts, not the doc.

Test plan

• Docs only; format clean; [skip ci].
• No broken intra-repo doc links (scanned).

Third and final tranche of the doc-voice/cleanup work: #549 (9 "why" docs), #550 (11 light-touch), this (18 archived).