Complete Designer autosave, recovery and external change handling

[ProGraMajster]

Summary

Completes reliable per-document Designer persistence, autosave recovery, crash recovery, and external-change handling without changing the canonical .mfdesign v1 format.

Autosave

• revision- and generation-aware scheduling with debounce and maximum interval
• no autosave while Designer transactions or normal Save are active
• atomic per-user recovery writes
• autosave never changes the dirty state or normal saved-revision marker

Recovery

• independent versioned recovery envelope with payload and metadata SHA-256 integrity checks
• saved and unsaved document identities with session/process isolation
• valid, obsolete, conflict, corrupt, and unsupported classification
• bounded discovery, cleanup containment, and safe data-only restore without executing user code

External changes

• exact-file FileSystemWatcher filtering and event coalescing
• bounded two-pass stable reads before parsing or hashing
• content-hash-based self-write suppression
• clean .mfdesign reload and explicit dirty-document conflict handling
• conservative, data-only .Designer.cs import with .mfdesign as the authoritative coalesced source

Reliability

• per-document generation tokens, write gates, and close/dispose guards
• stale callback and multi-document isolation
• atomic canonical .mfdesign and generated-file replacement at each individual file boundary
• recovery retention, cleanup limits, retry bounds, and rename identity handoff

Validation

• issue Complete Designer autosave, crash recovery and external-change detection #41 persistence/recovery tests: 138/138
• issue Add Designer undo/redo and transaction system #33 transaction tests: 67/67
• issue Add Designer copy, cut, paste and duplicate support #34 clipboard tests: 76/76
• Designer tests: 560/560
• Designer/runtime parity Add Designer/runtime layout parity test suite #42: 82/82
• Testing/TestHost Add headless TestHost and application UI testing infrastructure #64: 46/46
• all solution tests: 1549/1549
• Debug and Release solution builds: PASS, 0 warnings / 0 errors
• Android backend: Release build PASS; 72/72 tests
• Windows backend: Release build PASS; 7/7 tests
• cross-platform sample: Debug build PASS; 16/16 tests
• VSIX Debug and Release: PASS, validated packages
• DocFX metadata/site: PASS, 0 warnings / 0 errors
• release documentation script tests: 32/32 assertions
• versioned documentation archives: 4/4 validated against feature SHA 016240fb22b711a3b6e8d5379e2600ac409f261b
• git diff --check: clean
• manual Designer/Visual Studio UI smoke: pending; not inferred from automated checks

Closes #41