Add Designer transaction and undo/redo foundation

[ProGraMajster]

Summary

Adds the central Designer transaction and bounded undo/redo foundation for issue #33. The feature records typed model, property, layout, tree, rename, and selection changes per document; exposes Undo/Redo commands and toolbar state; integrates property-grid, collection-editor, outline, drag, resize, and root-layout mutations; and tracks the saved revision independently from the current history position.

Architecture

• Per-document DesignerTransactionManager and DesignerHistory with typed reversible changes.
• Explicit commit/rollback lifecycle, nested transaction merging, no-op suppression, replay suppression, configurable bounded retention, and save-marker invalidation when history branches or evicts the saved revision.
• Focused mutation snapshots use granular changes for normal edits; document-wide layout capture is reserved for root resize operations.
• Existing two-parameter DesignerSession construction remains compatible; new Designer APIs are additive and .mfdesign serialization is unchanged.

Designer integration

• Undo/Redo commands, toolbar buttons, localized captions, availability state, and per-document history switching.
• Atomic transactions across property-grid edits, collection editors, outline/tree operations, rename, drag/move/resize gestures, root layout changes, and document replacement.
• Selection, dirty state, save/reopen, code generation, reverse-parser, Form, and UserControl behavior retain regression coverage.

Reliability

• Partial mutation failures roll back atomically.
• Observer failures cannot strand active transactions, corrupt undo/redo stacks, leave stale gesture capture, or prevent session cleanup.
• Nested commit/rollback, reentrancy, save marker, history limits (1, 2, 500, and runtime changes), disposal, close, clear, eviction, and GC release paths have dedicated coverage.
• Stress coverage performs 1,000 property edits, 40 resize operations, 1,039 Undo operations, and 1,039 Redo operations with an exact serialization snapshot.

Validation

• Transaction-focused tests: 67/67.
• Designer tests: 342/342.
• Runtime/Designer parity suite (Add Designer/runtime layout parity test suite #42): 78/78.
• ModernFormsNext.Testing suite (Add headless TestHost and application UI testing infrastructure #64): 46/46.
• All tests: 1331/1331.
• Full Debug build: 0 warnings, 0 errors.
• Full Release build: 0 warnings, 0 errors.
• Android backend tests: 72/72; Android backend and smoke-test builds passed.
• Windows backend tests: 7/7.
• Cross-platform sample tests: 16/16; Windows and Android builds passed.
• VSIX Debug and Release builds validated the generated VSIX: 0 warnings, 0 errors.
• Canonical DocFX metadata/site builds: 0 warnings, 0 errors.
• Release documentation tests: 32/32; 4/4 local documentation archives validated against feature SHA 43313f4151bc9b25ad4a745fdd6fa6d2e7ec5ffd.
• git diff --check: clean.

Automated validation does not claim a manual Visual Studio Designer UI session or Android emulator/device run.

Follow-ups

Issues #34, #35, #36, #37, #39, and #41 remain separate follow-ups and are not implemented by this PR. This PR supplies the transaction/history foundation they can integrate with without changing their current status or target.

Closes #33