feat: add intro phase tests and fix death/damage handling during intro

- Move player.dead poll before intro early-return in update() so deaths
  triggered during the intro (e.g. fire ground) are handled correctly
- Add introPhase !== 'done' guard to Jacobot bug overlap callback so
  Jacobot cannot damage the frozen player during the camera intro
- Use player.unfreeze() instead of direct arcadeBody.setAllowGravity(true)
  for consistency with the freeze() call in create()
- Normalise single-quoted string literals to double quotes in intro code
- Add GameScene.introPhase.test.ts with 25 regression tests covering:
  (1) coin overlap blocked during every intro phase
  (2) intro state machine timing and phase transitions
  (3) player.unfreeze() + camera follow enabled exactly at intro end
  (4) death polled and handleDeath triggered during intro (fix regression)"

Agent-Logs-Url: https://github.com/DynamoDS/DynoDashGame/sessions/b3acf55e-e3c3-48f3-9c0e-4749293c972f

Co-authored-by: QilongTang <3942418+QilongTang@users.noreply.github.com>

Author: Copilot