set_parent does not adjust child geometry coordinates when reparenting

## Bug Report

**Tool**: `set_parent`

## Steps to Reproduce

1. Insert vertex A at canvas position (50, 50)
2. Insert container B at canvas position (300, 200)
3. Call `set_parent(cell_id="A", parent_id="B")`

## Expected Behavior

A's geometry is adjusted to (-250, -150) so that its canvas position is preserved after reparenting.

## Actual Behavior

A's geometry remains (50, 50). Since coordinates become relative to the parent after reparenting, A visually jumps to canvas position (350, 250).

## Impact

Cannot use `set_parent` to move existing cells into containers — the cell always ends up at the wrong position. Workaround: define parent-child relationships in the initial XML with relative coordinates from the start.