perf(macOS): scope workbench refresh and add FPS monitor

[1lck]

Summary

macOS workbench updates were fanning out through AppModel, so typing, caret movement, Git refresh, and language diagnostics rebuilt the whole window.

This PR keeps file watching and Git refresh, but applies each event to the surfaces that actually change:

• Isolate caret, selection, and Find in File on EditorChromeModel
• Isolate diagnostics on EditorDiagnosticsStore so LSP publish storms do not rebuild the workbench
• Make CodeEditorView incremental: dirty-range highlight, line-index edits, deferred decorations/LSP sync
• Rebuild Git tree status as a path index and refresh project-tree rows only when the snapshot changes
• Stop inactive project sessions from waking the window chrome
• Add a status-bar FPS monitor next to memory usage

How to verify

1. Open a medium or large file and type quickly / hold arrow keys. FPS in the status bar should stay closer to 60 on a 60 Hz display.
2. Edit an already-open file from another tool. The editor should still reload; the whole window should not hitch on every write.
3. Open Find in File and type. Only the find bar and match highlights should update.
4. Confirm Git color badges still update after workspace edits.

./scripts/test-macos.sh and ./scripts/verify-service-boundaries.sh were run locally.

Known follow-ups

• Markdown preview may not live-update after the first dirty keystroke, because document text no longer publishes on every already-dirty edit.
• Menu-bar Find Next/Previous enablement can lag because those items still read through AppModel.
• LSP log panes may wait for a session-state change before refreshing.