Record Windows beta3 preview candidate

[coneilen]

Summary

Records the exact unpublished 0.1.78-windows.beta3 candidate and Dev Box
handoff after the folder-picker and UIA qualification fixes merged.

Changes

• record candidate source/tag, package, payload, executable, provider, and
  toolchain identities
• record LFS-aware custody creation, verification, and offline restore with
  43/43 materialized files
• record the verified 8/8 Dev Box handoff inventory
• preserve beta1, beta2, and the unprefixed beta3 as immutable superseded
  evidence
• keep the six Dev Box-dependent gates open and preserve the 98/62/36 parity
  ledger

Test plan

RED: git show origin/main:investigation/windows-preview-release-plan.md | Select-String 7459aa9 -> 0 matches; replacement candidate absent
GREEN: Select-String -Path investigation\windows-preview-release-plan.md -Pattern 7459aa9 -> 4 matches; exact candidate and handoff recorded
REGRESSION: PowerShell count of investigation\ui-parity-matrix.md status rows -> TOTAL=98 VALIDATED=62 PARTIAL=36

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

Documentation-only change; macOS build/test/style commands were not run. The
candidate build, package verification, extracted setup verification, custody
restore, handoff inventory, and ledger-count commands are recorded in the
document.