Archive changelog sections before 0.41.0-alpha.3

[Kevinjohn]

Part of #1406 (T4). Merge predecessor: none.

• CHANGELOG.md keeps Unreleased and releases from 0.41.0-alpha.3 (the first GitHub release of the alpha line) upward; 107 older sections and their links move verbatim to CHANGELOG-ARCHIVE.md, with a pointer at the foot.
• Heading separators normalised to -; entry text untouched.
• documentation.test.ts gains an archive heading/reference parity check (mutation-checked: fails on a dropped or reordered reference).
• The development reference records one version bump per batch, each published as a GitHub release.

CHANGELOG.md is 1,541 lines, slightly over the plan's 1,500 target; keeping the 0.41.0-alpha.3 boundary took precedence.

Validation: integrated with the other Batch 1 branches on b04a75f5 (merge of origin/main da72918): pnpm run format:check, pnpm run docs:build (no drift) and pnpm run gate:server pass on Node 24.16.0. Independently reviewed before opening.

Merge order

1. Route support to Discussions and remove PR screenshots #1411, Add project status, comparison, testing and philosophy pages #1412, Archive changelog sections before 0.41.0-alpha.3 #1414 (independent)
2. Add a first-change path for contributors #1413 (after Archive changelog sections before 0.41.0-alpha.3 #1414), Restate the comment standard as numbered voice rules #1415, Reduce configuration and hide password MFA #1416
3. Rename SMALLSASS_ACCOUNT_ settings to CAPACITYLENS_ #1417 → Add an init command that writes the environment file #1418
4. Retire unused settings and derive engagement grouping #1419 → Move diagnostics to an Owner and Admin page #1420 → Reshape the documentation into four role guides #1421 → Use @/ for app imports that leave their folder #1422 → Consolidate server test scaffolding #1423 → Normalise server route handlers and reply messages #1424 → Apply the comment-voice rules and check them in the gate #1425 (Apply the comment-voice rules and check them in the gate #1425 also after Restate the comment standard as numbered voice rules #1415) → Name the tenant boundary account in code #1426

Merge predecessor for this PR: none.

[Kevinjohn]

Polish review: no P1/P2 findings.

• Content check: the sorted lines of the old CHANGELOG.md equal the union of the new CHANGELOG.md and CHANGELOG-ARCHIVE.md. Nothing is lost. The only differences are the heading separator (em dash to hyphen in archived headings) and the two pointer sentences.
• [Unreleased] still compares from v0.73.0-alpha.1. Nothing in scripts/, server/scripts/, .github/ or packaging/ parses the changelog.
• format:check, docs:build (no drift) and the documentation tests (14) pass.

Merge order for this batch: #1411, #1412, #1414 → #1413, #1415, #1416 → #1417 → #1418, #1419 → #1420 → #1421 → #1422 → #1423 → #1424 → #1425 → #1426. Predecessor: none. #1413 follows this PR, and both regenerate docs/reference/development.html.