## Why

The application currently applies all edits (transforms, attribute changes, prim creation/deletion, layer operations) directly to the live USD stage with no reversal mechanism, making it impossible to correct mistakes without closing and reopening the scene. Undo/redo is a foundational feature expected in any scene editor, and its absence is a critical usability gap.

## What Changes

- Introduce a `CommandHistory` class (command pattern) that records reversible `ICommand` objects.
- Wrap all mutating operations — prim create, prim delete, add/replace reference, transform gizmo drag, property panel TRS edits, arbitrary attribute edits, and layer management operations — in concrete `ICommand` subclasses with `Execute()` / `Undo()` pairs.
- Expose `Ctrl+Z` / `Ctrl+Y` (and `Ctrl+Shift+Z`) global hotkeys processed in the main loop.
- Integrate gizmo drag commits: the `TransformManipulator` currently emits continuous per-frame deltas; it will instead emit a single command on drag-end so that one undo step reverses the entire drag.
- Add **Edit → Undo / Redo** menu items with greyed-out state when the stack is empty.
- Clear the history stack on stage close / new stage open (invalid USD references cannot be safely replayed).

## Capabilities

### New Capabilities

- `command-history`: Core undo/redo stack — `ICommand` interface, `CommandHistory` manager, hotkey dispatch, and menu integration.
- `undoable-transform-edits`: Commands wrapping `UsdGeomXformCommonAPI` writes from both the viewport gizmo and the Property panel.
- `undoable-scene-edits`: Commands wrapping prim creation, prim deletion, add-reference, and replace-reference operations.
- `undoable-attribute-edits`: Command wrapping arbitrary `UsdAttribute::Set<T>` calls from `PropertyManager`.
- `undoable-layer-ops`: Commands wrapping `LayerManager` sublayer create, remove, reorder, and mute/unmute operations.

### Modified Capabilities

_(none — no existing spec-level requirements are being tightened or relaxed)_

## Impact

- **New file**: `src/core/CommandHistory.h/.cpp` — `ICommand`, `CommandHistory`.
- **New files**: `src/core/commands/` — one `.h/.cpp` pair per concrete command group.
- **Modified**: `src/ui/TransformManipulator.cpp` — capture drag-start state, emit command on drag-end instead of applying incremental deltas.
- **Modified**: `src/ui/PropertyPanel.cpp` — wrap TRS writes in commands.
- **Modified**: `src/ui/SceneHierarchyPanel.cpp` — wrap create/delete/ref operations in commands.
- **Modified**: `src/core/PropertyManager.cpp` — wrap `SetPropertyValue` in command.
- **Modified**: `src/ui/LayerPanel.cpp` — wrap layer CRUD in commands.
- **Modified**: `src/ui/Application.cpp` — add `CommandHistory` member, hotkey handling, Edit menu.
- **Modified**: `CMakeLists.txt` — add new source files.
- No new external dependencies.
