Introduces object interaction post-placement:
- Left-click selects the topmost object under the cursor (hit-test in world space)
- Selected object gets a strong outline + label
- Clicking empty space clears selection
- Works with pan/zoom
- Includes a fast way to create test objects: P enters placement mode, LMB places one valid object (same bounds/overlap rules), then returns to NAV
This POC proves the pick / selection primitive cleanly, independent of dragging.