feat: support shape changes on existing nodes (set_shape op)

[hugolytics]

Observed

Changing the shape: of an existing node via canvas_write_d2 is a no-op — the diff engine returns ops_applied: 0, warnings: [] and the node's Figma shapeType is unchanged.

Repro

1. Create a node with shape: diamond (e.g. "C2F indexing" { shape: diamond }).
2. Call canvas_write_d2 with the same full D2, only changing shape: diamond → shape: rounded_rectangle.
3. Result: ops_applied: 0. Canvas unchanged.

Expected

Shape property changes on existing shape_with_text nodes should be applied — either:

• emitted by the D2 diff engine as a set_shape op, or
• exposed as an explicit apply_patch op.

Root cause

• bridge/d2-diff.js doesn't treat shape: differences on existing nodes as a diff (only create/delete/rename/reparent/edge changes are emitted).
• plugin/code.js sets shape.shapeType only at node creation (lines ~521, ~1163, ~1395). No handler exists for a post-creation shape mutation.
• SHAPE_MAP already exists in bridge/d2-vocabulary.js — the D2 keyword ↔ Figma shapeType mapping is in place.

Feasibility

Figma plugin API allows mutating shape.shapeType on an existing ShapeWithTextNode, so this is a pure bridge/plugin-side change. No roundtrip or geometry implications (post-processor can reroute edges if shape bounds shift via canvas_relayout).

Proposed implementation

1. apply_patch op:

   { type: 'set_shape', node_id, after: 'ROUNDED_RECTANGLE' }
   

   Handler in plugin's ops.forEach that guards node.type === 'SHAPE_WITH_TEXT' and sets node.shapeType = op.after.
2. d2-diff.js: when an existing node's D2 shape: keyword differs from its current shapeType, emit a set_shape op (map via SHAPE_MAP).
3. Validator: reject set_shape on non-shape_with_text nodes (SECTION, CONNECTOR, etc.).

Workaround today

Delete the node (omit from target_d2) → re-add with the new shape in a second write. Loses position; must follow with canvas_relayout. Or change the shape manually in FigJam.