fix: rewrite seq/fec for federation-delivered packets

- Time-based dedup (2s TTL) replaces fixed-window dedup — consecutive
  senders with same seq numbers no longer collide
- Raw byte forwarding for federation local delivery (no re-serialization)
- Jitter buffer resets on large backward seq jumps (>100)
- recv_media skips malformed datagrams instead of returning connection-closed
- SIGTERM handler for clean QUIC shutdown on wzp-client
- JSONL event log infrastructure (--event-log flag) for protocol analysis
- FEC disabled on GOOD profile for federation debugging (fec_ratio=0.0)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/Cargo.toml

@@ -30,6 +30,7 @@ tower-http = { version = "0.6", features = ["fs"] }
 futures-util = "0.3"
 dirs = "6"
 sha2 = { workspace = true }
+chrono = "0.4"
 
 [[bin]]
 name = "wzp-relay"