fix: re-enable FEC + time-based dedup for federation

Restore fec_ratio=0.2 on GOOD profile. Time-based dedup (2s TTL) with
payload hash prevents consecutive sender collisions while still catching
multi-path duplicates. Verified: 6 consecutive senders across 2 relays,
0 decode errors, 0 drops, FEC active.

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

crates/wzp-proto/src/codec_id.rs

@@ -105,10 +105,10 @@ pub struct QualityProfile {
 }
 
 impl QualityProfile {
-    /// Good conditions: Opus 24kbps, FEC disabled for federation debugging.
+    /// Good conditions: Opus 24kbps, light FEC.
     pub const GOOD: Self = Self {
         codec: CodecId::Opus24k,
-        fec_ratio: 0.0,
+        fec_ratio: 0.2,
         frame_duration_ms: 20,
         frames_per_block: 5,
     };