canvas-populate: Add Content to Canvases

Read ../canvas/references/canvas-spec.md for the full JSON format before any edit. Read ../canvas/references/performance-guide.md for node limits and constraints.

General Workflow

For every add operation:

1. Read the target canvas JSON. Parse nodes and edges arrays.
2. Collect existing IDs to prevent collisions.
3. Generate a new ID: [type]-[slug]-[unix-timestamp].
4. Calculate position using the auto-positioning algorithm (see canvas orchestrator).
5. Spacing check: Verify the calculated position has at least 80px horizontal and 60px vertical gap from ALL existing content nodes. If not, shift the position until spacing is clear.
6. Append the new node to the nodes array (after any groups — z-index ordering).
7. Validate: Run python3 scripts/canvas_validate.py <path> — check for overlaps and node count.
8. Write the updated canvas JSON.