feat(quic): NAT traversal observability (rebind diagnostics + active sessions)

[louzt]

NAT Traversal Observability — 3 additive changes

SnapPipe v0.2.1 exposes three new lock-free observability surfaces for QUIC
path migration and NAT rebinding events.

D1: feat(quic): path-rebind diagnostic surface

Files: src/quic/rebind.rs (new), src/quic/mod.rs, tests/quic_rebind_diagnostics.rs

QUIC connections silently migrate paths (laptop Wi-Fi → 5G, carrier NAT rebinding).
Quinn does not surface a typed "path changed" event. The correct strategy is
periodic polling of ConnectionStats.

RebindDiagnostics exposes lock-free counters:

• poll_count — total polls performed
• rx_bytes / tx_bytes — UDP bytes received/transmitted (from stats.udp_rx.bytes)
• rtt_min_us / rtt_max_us — min/max RTT observed (from stats.path.rtt)
• rebind_count — incremented when local IP address hash changes between polls
• last_local_addr_hash — previous local IP hash for change detection

spawn_observer(conn, interval, diag, cancel) polls conn.stats() every
interval and calls diag.record_poll().

Quinn 0.11.9 API used:

• conn.stats() -> ConnectionStats where stats.path: PathStats (field, not method)
• stats.udp_rx.bytes / stats.udp_tx.bytes: u64
• stats.path.rtt: Duration
• conn.local_ip() -> Option<IpAddr>

D2: feat(session): NAT rebinding tolerance observability

Files: src/session.rs

SessionMetrics exposes 5 atomic handshake outcome counters:

• successful — ticket valid, issuer trusted, subject matched
• expired_or_invalid — ticket expired or signature invalid
• issuer_not_trusted — issuer not in trust store
• subject_mismatch — subject did not match
• other_errors — protocol error or I/O failure

Complements RebindDiagnostics (transport layer) with handshake outcome
observability (session layer).

The module docstring now includes a "NAT rebinding tolerance" section
documenting how QuicTransportProfile::keep_alive_interval_ms and
RebindDiagnostics together give operators visibility into path migrations.

D3: feat(metrics): relay-side active-sessions counter

Files: src/relay/mod.rs

Relay now tracks active_sessions: Arc<AtomicU64>. The counter is
incremented before handle_connection starts and decremented when it
finishes (including early rejection paths: trust rejection, rate limiting).

Relay::active_sessions() -> u64 exposes the current count.

Scope boundary

• All changes are additive — no existing APIs modified
• SessionMetrics is declared but not yet instantiated (increment site is
  a future change to the session handshake)
• No disk I/O, no external dependencies, no TLS changes

Validation

cargo test --locked              # 75 tests
cargo clippy --locked --all-targets -- -D warnings  # 0 warnings

Dependencies

• D3 depends on C1 (feat(relay): real Quinn socket listener, already merged)
• D1 and D2 are independent of each other and of D3

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📥 Commits

Reviewing files that changed from the base of the PR and between ad3df11 and 0d5dc6e.

📒 Files selected for processing (6)

• docs/OPERATIONAL-DEPLOYMENT.md
• src/quic/mod.rs
• src/quic/rebind.rs
• src/relay/mod.rs
• src/session.rs
• tests/quic_rebind_diagnostics.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/nat-traversal-observability

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