refactor: Add built-in observability metrics for hydration and boot

[joseph0926]

Problem

Claims like "80-85% hydration success rate" lack verifiable measurement in user apps.

Proposed Solution

1. Record metrics to window.FIRSTTX_METRICS:
2. Hydration failure/reset count
   • Snapshot size
   • Capture duration
   • Boot application status
3. Visualize in devtools
4. Provide users with measurable data instead of claims