feat: --debug-tap for relay packet header logging

Adds --debug-tap <room> flag (or debug_tap in TOML config) that logs
every media packet's header metadata passing through a room. Use '*'
for all rooms.

Output (via tracing target "debug_tap"):
  TAP room=... dir=in addr=... seq=31 codec=Opus24k ts=520
      fec_block=5 fec_sym=1 repair=false len=65 fan_out=1

Shows: direction, source address, sequence number, codec ID, timestamp,
FEC block/symbol, repair flag, payload size, and fan-out count.
No decryption needed — headers are not encrypted.

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

crates/wzp-relay/src/config.rs

@@ -63,6 +63,9 @@ pub struct RelayConfig {
     /// Federation peer relays.
     #[serde(default)]
     pub peers: Vec<PeerConfig>,
+    /// Debug tap: log packet headers for matching rooms ("*" = all rooms).
+    /// Activated via --debug-tap <room> or debug_tap = "room" in TOML.
+    pub debug_tap: Option<String>,
 }
 
 impl Default for RelayConfig {
@@ -82,6 +85,7 @@ impl Default for RelayConfig {
             ws_port: None,
             static_dir: None,
             peers: Vec::new(),
+            debug_tap: None,
         }
     }
 }