Record unpublished Windows beta21 preview candidate

[coneilen]

Summary

Record the immutable, unpublished Windows beta21 preview candidate and its
LFS-aware Dev Box handoff.

Beta20 failed and is superseded by its Dev Box findings and review follow-ups,
fixed in #679–#685 and #687–#693:

• terminal: mouse selection and copy, mouse forwarding, F6/F10, AltGr, paste
  bounds, cleanup safety, Close Pane;
• the sidebar;
• zmx unknown liveness in the shell and in shared GraphcodeKit/graphcoded.

This is the first candidate since beta1 whose daemon and CLI changed.

Approval A keeps fixture-only Worktrees reclaim. All runtime qualification
gates remain open.

Changes

• Record source/tag/package/payload/executable/provider/toolchain identities for
  0.1.78-windows.beta21 at d45a7bed1736368e99b03218e1690042226fea8b.
• Record the fixes, the local windows-shell and swift-production validation,
  the live UIA status-line race and its passing rerun, and a harness-cleanup
  gap (graphcode-windows/shell-sessions left by the smoke gate).
• Record custody, the offline restore (43/43 LFS) and the 8/8 handoff.
• Point the current-candidate references (Exact artifact gate, publication
  record, plan binding, Windows first-run onboarding blocks daemon connection; main window shows "Disconnected" and dialog has no UIA tree #556 orchestration item) at beta21.
• Move beta20 to history. Beta1–beta20 and their handoffs are unchanged.

Test plan

RED: Tools\windows\validate.ps1 -Task windows-shell -SkipTrayLive -ShellValidationRoot C:\v7 at d45a7be -> VALIDATE_EXIT=1; live UIA gate read the status line 250 ms after the stale Move command and found an unrelated loop-launch status; all other sections passed
GREEN: Tools\windows\validate.ps1 -Task windows-shell -ShellPart integration -SkipTrayLive -ShellValidationRoot C:\v7 at d45a7be -> VALIDATE_EXIT=0 including the live UIA gate, scrubbed 5/5 and REAL_DAEMON_ROUNDTRIP: PASS
REGRESSION: Tools\windows\validate.ps1 -Task swift-production at d45a7be -> VALIDATE_EXIT=0, shared Swift 136 tests 0 failures, daemon/CLI bootstrap smoke passed; release.ps1 -Tag 0.1.78-windows.beta21 -> repository verify 1 PASS, extracted setup verify 1 PASS, payload 50, packaged scrubbed startup 5/5

• The first full run also passed the unit and integration sections: 55/55
  sections, App 1114/1114, and the bare-name zmx smoke. Only the live UIA gate
  failed; the rerun covers the integration part.
• The RED line is a local gate timing failure, kept rather than replaced. I did
  not prove it is not a product interaction: the stale Move check reads the
  status line after a fixed 250 ms. CI integration was green at Windows shell: treat zmx err= rows other than ConnectionRefused as unknown #691's and
  GraphStore: act on unknown zmx session liveness by deferring (B2) #693's exact heads.
• GraphcodeKit: treat zmx err= rows other than ConnectionRefused as unknown (launcher layer) #692 and GraphStore: act on unknown zmx session liveness by deferring (B2) #693 ran the macOS and Linux jobs green at their exact heads. macOS
  make test/make check were not run locally.
• validate.ps1 -Task packaging was not run locally. The generated Setup script
  is byte-identical to beta20, and the packaging real-products job was green at
  GraphStore: act on unknown zmx session liveness by deferring (B2) #693's exact head.
• Parity: 57 Validated / 41 Partial, total 98, unchanged from beta20.
• Source custody Create/Verify passed with 43 objects for 43 files. The
  file-only offline restore gave exact detached HEAD/tag, zero remotes, clean
  status and 43/43 materialized LFS files.
• Handoff inventory: 8/8 verified; hashes.sha256 SHA-256
  3294bf1f54d3ac9bb92c6a4ba94148200bcf108d19040a8cbe7f5a2a7fcef2a2.
• Not claimed: physical keyboard or mouse behaviour, zmx liveness under real
  load, Dev Box results, backend, destructive, Nod, dump or publication
  evidence.
• No tag or release was pushed or published.

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test)
• Code follows the existing style (make check)
• I added the test/contract before the implementation and observed the intended RED failure