UrpCameraStackBinder rebinds every ScreenSpace-Camera canvas in a bound
scene to the persistent UI camera (unconditional overwrite, to fix stale/
wrong baked camera refs that cause "UI camera is blind"). That hijacked
canvases intentionally bound to another camera (e.g. a background rendered
by the scene's own base camera).

- New marker component SceneV2KeepCanvasCamera; binder skips canvases that
  carry it, leaving their worldCamera untouched. Default behavior unchanged
  for unmarked canvases.
- Marker only prevents the overwrite — it does not assign a camera; the
  canvas's Render Camera must still be set in the scene.
- Docs: README §14 (render context & camera stacking), CLAUDE.md render
  bullet, progress.md maintenance note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>