feat(relay): record per-leg frame progress for stall triage - #197

Refs #161. Built on merged #196 for per-attempt correlation. This is monitoring-only; the underlying #161 root cause remains unknown.

Monitoring change

• Add opt-in, per-leg opaque WebSocket ingress frame/byte counts and completed downstream socket-send counts plus first/last UTC frame times to Relay stream_close events.
• Keep default diagnostic logging off and avoid per-frame wall-clock sampling when it is disabled.
• Add success and stalled-forwarding fixtures; document four-locales interpretation and privacy limits.

This supports investigation if the public-path stall recurs. A completed socket send is not proof of peer delivery, and these counters cannot determine whether Cloudflare, OCI, or a transit hop is at fault. No production service, security setting, or existing unmerged #161 observability worktree was changed.

Verification

• Full local suite: 517 run, 506 passed, 11 skipped (Python 3.14.7, websockets 17.1).
• Paired-stream and forward-timeout metadata/privacy tests pass.
• Documentation i18n checks pass after rebase.

Existing sqlite ResourceWarning in test_control_backup_snapshot.py remains unrelated.