Fix scene lifecycle issues and make custom effect loading more resilient

[tusharmalpani20]

This PR fixes a handful of issues I found while reviewing the animation engine and custom effect loading.

The main changes are:

• Broken custom effects are now skipped with a clear warning instead of preventing TTE from starting.
• A failed custom effect can no longer leave behind a partially registered command.
• Custom effect parser setup is only called once.
• Looping scenes now emit SCENE_COMPLETE once per completed cycle.
• Synced looping scenes keep their frames when no motion path is active.
• Empty or invalid symbol sequences now produce a clear error.
• Duplicate scene IDs are rejected instead of silently replacing an existing scene.
• The Beams effect no longer depends on duplicate scene replacement.
• The application guide and changelog have been updated.

I also added regression tests for these cases.

Testing

• 94 CLI and animation tests passed.
• 804 Beams configurations passed.
• 1,444 smoke tests passed.
• Ruff checks passed.
• The strict documentation build passed.
• The UV lockfile remains unchanged.