docs: record backend perf-gate green evidence in MASTER/roadmap (T79.2)

## Goal
Document that `verify-backend-perf-leak-gates.ps1` passes; do not claim production capacity.

## Acceptance
- [ ] MASTER or roadmap notes gate script + evidence boundary
- [ ] Explicit: microbench/behavior only, not leak/capacity proof

## Super
E

## Delivery Batch
B1