fix(audit): address C2, C3, M4, M5 from 2026-05-25 audit

C2: Add EncryptingTransport wrapper — all media I/O now goes through
ChaChaSession encrypt/decrypt before hitting the QUIC datagram path.
cli.rs run_live/run_silence/run_file_mode accept Arc<dyn MediaTransport>
and receive a wrapped transport after the handshake.

C3: Wire VideoScorer::observe() into both plain and trunked forwarding
loops in room.rs. Packets from participants with Abusive verdict are
dropped before forwarding. last_bwe_kbps tracked from quality reports.

M4: Widen FEC repair symbol index from u8 to u16 throughout
(FecEncoder::generate_repair, FecDecoder::add_symbol, all call sites in
call.rs, bench.rs, pipeline.rs, wzp-android). Eliminates theoretical
wrapping when num_source + repair_count > 255.

M5: Track last_encrypt_timestamp in ChaChaSession. debug_assert in
encrypt() that timestamp is non-decreasing across calls (including post-
rekey). complete_rekey() explicitly preserves last_encrypt_timestamp to
prevent accidental timestamp reset regressions.

583 tests passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/wzp-relay/src/pipeline.rs

@@ -111,7 +111,7 @@ impl RelayPipeline {
         let header = &packet.header;
         let _ = self.fec_decoder.add_symbol(
             (header.fec_block & 0xFF) as u8,
-            (header.fec_block >> 8) as u8,
+            header.fec_block >> 8,
             header.is_repair(),
             &packet.payload,
         );