fix: FEC decoder resets stale blocks — fixes consecutive federation connects

When a new sender reuses the same block_id values as a previous sender,
the FEC decoder was silently dropping all data because blocks were marked
as "already decoded". Now blocks older than 2 seconds are automatically
reset when new data arrives for them.

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

crates/wzp-fec/src/decoder.rs

@@ -1,6 +1,7 @@
 //! RaptorQ FEC decoder — reassembles source blocks from received source and repair symbols.
 
 use std::collections::HashMap;
+use std::time::Instant;
 
 use raptorq::{EncodingPacket, ObjectTransmissionInformation, PayloadId, SourceBlockDecoder};
 use wzp_proto::error::FecError;
@@ -9,6 +10,9 @@ use wzp_proto::FecDecoder;
 /// Length prefix size (u16 little-endian), must match encoder.
 const LEN_PREFIX: usize = 2;
 
+/// Decoded blocks older than this are eligible for reuse by a new sender.
+const BLOCK_STALE_SECS: u64 = 2;
+
 /// State for one in-flight block being decoded.
 struct BlockState {
     /// Number of source symbols expected.
@@ -21,6 +25,8 @@ struct BlockState {
     decoded: bool,
     /// Cached decoded result.
     result: Option<Vec<Vec<u8>>>,
+    /// When this block was last decoded (for staleness check).
+    decoded_at: Option<Instant>,
 }
 
 /// RaptorQ-based FEC decoder that handles multiple concurrent blocks.
@@ -58,6 +64,7 @@ impl RaptorQFecDecoder {
             symbol_size: self.symbol_size,
             decoded: false,
             result: None,
+            decoded_at: None,
         })
     }
 }
@@ -74,9 +81,21 @@ impl FecDecoder for RaptorQFecDecoder {
         let block = self.get_or_create_block(block_id);
 
         if block.decoded {
-            // Already decoded, ignore additional symbols.
+            // If the block was decoded recently, skip (normal duplicate).
+            // If it's stale (>2s), a new sender is reusing this block_id — reset it.
+            if let Some(at) = block.decoded_at {
+                if at.elapsed().as_secs() >= BLOCK_STALE_SECS {
+                    block.decoded = false;
+                    block.result = None;
+                    block.decoded_at = None;
+                    block.packets.clear();
+                } else {
                     return Ok(());
                 }
+            } else {
+                return Ok(());
+            }
+        }
 
         // Data should already be at symbol_size (length-prefixed and padded by the encoder).
         // But if caller sends raw data, pad it.
@@ -132,6 +151,7 @@ impl FecDecoder for RaptorQFecDecoder {
 
                 let block = self.blocks.get_mut(&block_id).unwrap();
                 block.decoded = true;
+                block.decoded_at = Some(Instant::now());
                 block.result = Some(frames.clone());
                 Ok(Some(frames))
             }