Build a per-post and per-file decision trace for extension debugging

[adhamhaithameid]

Summary

Add a decision trace so the extension can explain why it injected or rejected a button or flag.

Why

The current behavior is strong, and the next risk is accidental regression during future hardening. Debugging gets much cheaper when each decision is explainable.

Scope

• record why a file was considered downloadable or rejected
• record why a post received a comment flag, edited flag, both, or none
• expose trace data through the existing debug surface in a readable format
• add tests for trace generation and stability

Effect

Faster debugging, lower regression cost, and better confidence when touching selectors or scoring.