feat(3f): add bridge facilitator metrics

[Trynax]

Summary

I noticed the shared metrics seam was already in place and used by the other solvers, but the 3F bridge facilitator still wasn’t exporting any solver-local metrics, so I picked that up here.

What changed

• add discovery, offer-cache, offer-submit, and redeem-flow metrics
• wire metrics into the 3F solver and redeemer paths
• add unit coverage for the new metrics helpers
• sync the 3F plan note now that the metrics seam is in use