Refactor: generalize CanvasHistory into BoundedHistory<T> for image undo

[GamalAnwar]

Summary

Follow-up to #6. The annotation undo/redo now lives in CanvasHistory, but the image-level undo/redo was still two loose @State [NSImage] arrays (imageUndoStack/imageRedoStack) with inline push / cap-at-20 / pop logic in EditorView — duplicating the exact rules #6 just extracted.

This generalizes CanvasHistory into BoundedHistory<Snapshot> and uses it for both:

typealias CanvasHistory = BoundedHistory<CanvasState>          // unchanged usage/tests from #6
@State private var imageHistory = BoundedHistory<NSImage>(limit: 20)

EditorView's image stack handling collapses from inline array bookkeeping to:

private func pushUndoImage() { imageHistory.record(workingImage); pushUndo() }
// undo(): if let image = imageHistory.undo(current: workingImage) { workingImage = image }
// redo(): if let image = imageHistory.redo(current: workingImage) { workingImage = image }
// hasEdits: ... || imageHistory.canUndo

Scope / safety

• Behavior-preserving. Image and canvas histories are still popped independently in undo()/redo() — image is best-effort (if let), canvas keeps its guarded return — matching the prior ordering exactly.
• CanvasHistory is now a typealias, so Refactor: extract annotation undo/redo into CanvasHistory #6's call sites and CanvasHistoryTests are untouched.
• One repository = one history type; no near-duplicate stack code remains.

Tests

BoundedHistoryTests (4 cases) pins the generic behavior with Int (mirroring the NSImage use): record/undo/redo round-trip, record-clears-redo, nil-on-empty, and cap-drops-oldest at limit 20. CanvasHistoryTests (7) continues to cover the CanvasState instantiation.

🤖 Generated with Claude Code

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