canvas-layout: Re-Layout Canvas Nodes

Read ../canvas/references/layout-algorithms.md for algorithm details and selection guide. Read ../canvas/references/performance-guide.md for node limits.

Workflow

1. Identify target canvas: Use the active canvas or ask which one (/canvas list to show options).
2. Select algorithm: Use the user-specified algorithm, or run auto detection.
3. Confirm before applying: Show what will happen: "Will apply [algorithm] layout to [canvas] ([N] nodes, [M] groups). Create backup? [Y/n]"
4. Run the layout script:

   python3 scripts/canvas_layout.py [canvas_path] [algorithm] [options]
   

5. Report results: "Moved [N] of [M] nodes. [G] groups refitted. Backup at [path].bak"
6. Validate: Run python3 scripts/canvas_validate.py [canvas_path] to confirm valid output.