Circuit checkpoint / version history system

[darktorres]

Description

Add a non-destructive versioning system (like Git for circuits) that lets users create named checkpoints, browse history, and branch into alternative designs without losing previous work.

Features

• Manual named checkpoints ("before adding ALU", "working version")
• Auto-checkpoints at configurable intervals
• Visual timeline of checkpoints
• Restore any checkpoint without losing later ones

Acceptance Criteria

• Intuitive UI for creating and browsing checkpoints
• Performance is not impacted during normal editing
• Storage is efficient (compression, deduplication of unchanged elements)

Implementation Guide

1. Checkpoint storage

• Each checkpoint is a serialized snapshot of the circuit (use existing Serialization::serialize())
• Store as compressed blobs (qCompress) in a sidecar file (e.g., .panda.history) or embedded in the .panda metadata
• Delta compression: store only changed elements relative to previous checkpoint (optional optimization)

2. Checkpoint manager

• Create CheckpointManager class owned by WorkSpace
• Methods: createCheckpoint(name), restoreCheckpoint(index), listCheckpoints()
• Auto-checkpoint: connect to Scene::circuitHasChanged with a debounce timer

3. UI

• Add a checkpoint panel (dockable widget or sidebar)
• Show timeline with checkpoint names and timestamps
• Double-click to restore, right-click for rename/delete
• Add toolbar button for quick checkpoint creation

4. Integration with undo stack

• Checkpoints are independent of QUndoStack — they capture full state, not incremental commands
• Restoring a checkpoint clears the undo stack and loads the snapshot

Key files

• App/Scene/Workspace.h/.cpp — file management, save/load
• App/IO/Serialization.h/.cpp — circuit serialization
• App/UI/MainWindow.cpp — UI integration
• New files: App/Scene/CheckpointManager.h/.cpp, App/UI/CheckpointPanel.h/.cpp