feat: add plugin architecture with bidirectional clip sync

[fuzzzerd]

Summary

• Add extensible plugin system with GPL plugin exception clause permitting non-GPL plugins
• Introduce IClipEditor abstraction for clip-type-agnostic bidirectional sync between structured editors and plugins
• Reorganize menu into File/Edit/Plugins with version in status bar

Plugin Architecture

• SharpFM.Plugin contract library: IPanelPlugin, IPluginHost, IClipEditor, PluginKeyBinding, PluginMenuAction
• Plugin discovery and loading from plugins/ directory at runtime
• Plugin Manager UI for install from file and removal
• Keyboard shortcut registration and custom menu actions for plugins
• Two bundled plugins: Clip Inspector (sample) and XML Viewer (replaces built-in)

Bidirectional Sync

• IClipEditor interface with ScriptClipEditor, TableClipEditor, FallbackXmlEditor implementations
• Each editor handles its own debounced change detection (500ms)
• Generation counter prevents editor-host sync loops (zero cost, monotonic)
• Origin tagging on ClipContentChanged events enables multi-plugin coordination
• FmField now implements INotifyPropertyChanged for live table editor tracking
• Table FromXml uses diff/patch to preserve UI state (selection, scroll)

Menu Reorganization

• File: New Script, New Table, Open Folder, Save All, Exit
• Edit: Paste from FileMaker, Copy to FileMaker, Copy as C# Class
• Plugins: loaded plugins with keyboard shortcuts, Manage Plugins...
• Version moved to status bar

Test plan

• 198 tests pass (dotnet test)
• Script clip: edit in script editor -> XML viewer updates. Edit XML -> script updates.
• Table clip: add/edit field -> XML viewer updates. Edit XML -> table patches.
• Fallback clip: bidirectional XML sync
• Plugins menu lists loaded plugins with gesture hints
• Manage Plugins dialog: install from file, remove
• App runs fine with empty plugins/ directory