Harden monitor lifetime and mode handling

[itsmikethetech]

Summary

• isolate monitor state and cleanup by monitor object lifetime
• validate monitor, adapter, mode, and swap-chain callback state
• correctly handle inactive paths and target-mode buffer contracts
• restore release-gated SignPath signing for the x64 and ARM64 VDD packages

Validation

• actionlint .github/workflows/ci-validation.yml
• git diff --check
• GitHub Actions run 35429064594: x64 and ARM64 Release builds passed; each uploaded the expected unsigned VDD package.
• SignPath was intentionally skipped for this pull-request build. It runs only for a v* tag or a manual dispatch with the sign checkbox enabled.