Add Designer copy, cut, paste and duplicate support

[ProGraMajster]

Summary

Add safe detached Designer clipboard operations for controls and complete subtrees.

Operations

• Copy
• Cut
• Paste
• Duplicate

Architecture

• detached, deterministic, versioned JSON payload (Format = ModernFormsNext.Designer, Version = 1)
• no live control or source-document references
• safe data-only type and property reconstruction
• deterministic document-wide name remapping
• same-session cross-document copy that survives closing and collecting the source document
• atomic transaction and Undo/Redo integration from Add Designer undo/redo and transaction system #33

Designer integration

• selection restoration
• Document Outline refresh
• Property Grid refresh
• toolbar commands and enabled states
• Ctrl+C / Ctrl+X / Ctrl+V / Ctrl+D, while active Property Grid text editors retain normal text clipboard behavior
• deterministic code generation
• .mfdesign save/reopen and reverse-parser compatibility

Security

• no arbitrary CLR deserialization or assembly-qualified type resolution from clipboard data
• no assembly loading
• no user control constructor execution
• no delegates, native handles, dispatchers, Skia objects, backend objects, or event subscriptions in the payload
• strict schema, identifier, canonical value, size, node-count, and depth validation before mutation

Validation

• Add Designer copy, cut, paste and duplicate support #34 clipboard tests: 76/76
• Add Designer undo/redo and transaction system #33 transaction tests: 67/67
• Designer tests: 422/422
• parity Add Designer/runtime layout parity test suite #42: 82/82
• Testing Add headless TestHost and application UI testing infrastructure #64: 46/46
• all tests: 1411/1411
• Debug: PASS, 0 warnings / 0 errors
• Release: PASS, 0 warnings / 0 errors
• Android backend tests: 72/72; Debug/Release builds PASS
• Windows backend tests: 7/7; Debug/Release builds PASS
• cross-platform sample tests: 16/16; Windows/Android builds PASS
• VSIX Debug/Release: PASS and package validation PASS
• DocFX metadata/site: PASS, 0 warnings / 0 errors
• release documentation tests: 32/32 assertions
• release documentation archives: 4/4 validated
• 331-node Copy/Paste/Undo/Redo stress: deterministic and within the 10-second limit
• git diff --check: clean

No manual Android device/emulator or interactive Designer UI smoke result is claimed; platform and UI evidence above is automated.

Scope

• single selection only; Add Designer multi-selection and group editing #35 remains future work
• runtime/operating-system Clipboard Add cross-platform Clipboard and runtime Drag & Drop infrastructure #57 is not implemented
• richer event workflow Complete Designer event-handler generation and code navigation #37 is not implemented

Closes #34