Add global undo/redo system with CDC deduplication (Ctrl+Z / Ctrl+Y)

[Z3roCo0l]

Summary

MeshLab has no built-in undo system (see #376). This adds a practical, constrained undo/redo mechanism that works across all filters and edit tools without refactoring MeshLab's internal architecture.

• Ctrl+Z / Ctrl+Y — undo / redo
• Available in the Edit menu
• Supports up to 50 undo levels
• Covers all filter operations and the Lasso Cut tool

Design Approach

This is intentionally not a full architecture-level undo framework. Instead, it takes mesh state snapshots at well-defined operation boundaries (before each filter execution and before lasso cut operations). The approach coexists with the current architecture without modifying action dispatch, state management, or plugin interfaces.

CDC (Content-Defined Chunking) Deduplication

To make 50 undo levels practical, the system uses content-defined chunking instead of storing full mesh copies:

1. Serialize — mesh data (vertices, faces, OCF attributes) → deterministic byte buffer (no pointers, compacted indices)
2. Chunk — Gear hash rolling function splits buffer at content-dependent ~4KB boundaries
3. Deduplicate — FNV-1a hash identifies identical chunks; shared reference-counted ChunkStore avoids duplication

	Full mesh copy	CDC dedup
10 undo levels (1M vert mesh)	~920 MB	~93 MB
50 undo levels (1M vert mesh)	N/A	~130 MB

Serialization

The serializer handles all CMeshO components:

• Vertex core: coords, normals, color, quality, flags
• Face core: vertex indices (compacted), normals, flags
• OCF attributes: mark, texcoord, curvature direction, radius, quality, color, wedge texcoords
• Adjacency (VF/FF) is not serialized — rebuilt via UpdateTopology on restore
• Camera shot, transformation matrix, bounding box, texture strings

Limitations

• Snapshot-based: captures full mesh state, not individual operations
• Custom per-vertex/per-face attributes (PointerToAttribute) are not serialized (uncommon in standard workflows)
• No per-layer undo — operates on the current mesh only

Files Changed

• src/meshlab/mesh_undo_stack.h — Complete undo system (CDC pipeline + MeshUndoStack class, ~870 lines, all in single header)
• src/meshlab/mainwindow.h — Undo stack member, Ctrl+Z/Y actions, slots
• src/meshlab/mainwindow_Init.cpp — Menu entries, keyboard shortcuts
• src/meshlab/mainwindow_RunTime.cpp — Pre-filter snapshot hook, undo/redo implementation
• src/meshlabplugins/edit_cut/edit_cut.cpp — Pre-cut snapshot via QMetaObject::invokeMethod (no linker dependency on meshlab.exe)

Test plan

• Open mesh, apply Laplacian smooth, Ctrl+Z — mesh restores
• Ctrl+Y — redo works
• Use lasso cut (Q), Ctrl+Z — mesh restores with correct topology
• Perform 55 operations, verify oldest undo is dropped (50 level cap)
• Monitor memory with multiple undo levels — should be ~85-90% less than full copies
• Build on Windows, Linux, macOS

Note: This is PR 3 of 3 (split from #1638). Depends on #1639 (global params) and #1640 (lasso cut).