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-proto/src/traits.rs

@@ -81,7 +81,7 @@ pub trait FecEncoder: Send + Sync {
     ///
     /// `ratio` is the repair overhead (e.g., 0.5 = 50% more symbols than source).
     /// Returns `(fec_symbol_index, repair_data)` pairs.
-    fn generate_repair(&mut self, ratio: f32) -> Result<Vec<(u8, Vec<u8>)>, FecError>;
+    fn generate_repair(&mut self, ratio: f32) -> Result<Vec<(u16, Vec<u8>)>, FecError>;
 
     /// Finalize the current block and start a new one.
     /// Returns the block ID of the finalized block.
@@ -100,7 +100,7 @@ pub trait FecDecoder: Send + Sync {
     fn add_symbol(
         &mut self,
         block_id: u8,
-        symbol_index: u8,
+        symbol_index: u16,
         is_repair: bool,
         data: &[u8],
     ) -> Result<(), FecError>;