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-client/src/bench.rs

@@ -170,7 +170,7 @@ pub fn bench_fec_recovery(loss_pct: f32) -> FecResult {
 
         // Collect all symbols: source + repair
         struct Symbol {
-            index: u8,
+            index: u16,
             is_repair: bool,
             data: Vec<u8>,
         }
@@ -180,7 +180,7 @@ pub fn bench_fec_recovery(loss_pct: f32) -> FecResult {
             // For add_symbol we need to provide the raw data; the decoder pads internally
             total_source_bytes += sym.len();
             all_symbols.push(Symbol {
-                index: i as u8,
+                index: i as u16,
                 is_repair: false,
                 data: sym.clone(),
             });