fix(core): remaining 15 sweep lows — client, clipboard, C ABI (v10), FEC, GSO, GTK

Second and final batch from the 2026-07-20 core-sweep lows:

client:
- connect() timeout now sets `quit` before shutdown, so a handshake that
  completes after the deadline closes with QUIT_CLOSE_CODE instead of
  leaving the host lingering (virtual display up) for a reconnect that
  never comes.
- probe_result: saturating_add on the wire-supplied wire_packets +
  send_dropped counters (debug-build overflow panic / release wrap).
- the standing-latency bleed no longer sets flush_in_window: that flag
  is the ABR's SEVERE (×0.7) verdict, and the bleed fires only on
  provably loss-free windows the controller itself scores as fine.

clipboard:
- fetch_cancels pruned on every new fetch (was: one dead oneshot per
  paste for the session).
- serve chunks gated on a parked waiter + capped at CLIP_FETCH_CAP with
  an Error event (was: unbounded silent accumulation under any req_id).
- serve_inbound park bounded by FETCH_STALL_SECS + send.stopped() (was:
  an unanswered FetchRequest parked the task, waiter, and bi-stream
  forever; ~100 of them exhaust the connection's bidi budget).

C ABI (ABI_VERSION 9 → 10, header regenerated, additive only):
- new punktfunk_connection_clock_offset_now_ns — the LIVE re-synced
  offset (Swift/Kotlin latency math read the frozen connect-time value
  ~40ms wrong after a wall-clock step).
- to_config: checked u64→usize narrowing of max_frame_bytes (32-bit
  armeabi truncated >4GiB to a plausible residue).
- host_poll_input: no &mut held across the embedder callback (re-entry
  aliased it — UB under noalias); mid-drain callback clears now stick.
- next_audio_pcm: DTX (empty) payloads skipped — decode synthesized
  120ms of concealment per 5ms slot and grew the playout ring forever.
- next_clipboard releases the parked payload on an empty poll (a one-off
  50MiB paste stayed resident all session).
- frames_dropped / wants_decode_latency write their documented 0/false
  defaults before the NULL-handle check.
- gamepad constant docs match pick_gamepad() reality (DualSenseEdge/
  SwitchPro landed; DualSense/DS4 honored on Windows UMDF too).

FEC:
- gf8 reconstruct/reconstruct_into reuse the (k,m) codec cache like
  encode_into (was: fresh 230×200 generator + decode inversion per
  lossy block on the pump thread).
- vendored fec-rs: the 8 safe wrap_mul_slice shims assert equal lengths
  (x86 SIMD callees bound stores on input.len() — safe-code OOB write);
  ReconstructShard's safety contract gains the len()==get().len()
  clause and reconstruct_internal sizes raw slices from the slice.

transport/GTK:
- Linux GSO super-buffer capped at the real UDP payload ceiling (65487)
  not 65535 — seg sizes ≥1024 could EMSGSIZE and latch GSO off
  process-wide, blamed on the network.
- GTK settings dialog no longer rewrites an unlisted-but-valid stored
  gamepad preference to "auto" on close.

Already fixed on main (verified stale, skipped): the reassembler FEC
ceiling, wants_decode_latency's third term, request_probe rollback +
the generalized probe watchdog.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-core/src/fec/gf8.rs

@@ -86,8 +86,18 @@ impl ErasureCoder for Gf8Coder {
             // No FEC: every original must already be present.
             return collect_originals(received, data_count);
         }
-        let rs = ReedSolomon::new(data_count, recovery_count)
-            .map_err(|_| FecError::Config("invalid GF(2^8) shard counts"))?;
+        // Same (k, m)-keyed cache as `encode_into`: a fresh ReedSolomon per lossy block costs a
+        // full generator build (k×2k Gauss-Jordan + total×k×k multiply) on the real-time pump
+        // thread AND forfeits the instance's decode-matrix cache for stable loss patterns.
+        let mut guard = self.rs.lock().unwrap_or_else(|p| p.into_inner());
+        let cached =
+            matches!(&*guard, Some((ck, cm, _)) if *ck == data_count && *cm == recovery_count);
+        if !cached {
+            let rs = ReedSolomon::new(data_count, recovery_count)
+                .map_err(|_| FecError::Config("invalid GF(2^8) shard counts"))?;
+            *guard = Some((data_count, recovery_count, rs));
+        }
+        let rs = &guard.as_ref().expect("cache populated above").2;
         rs.reconstruct_data(received)
             .map_err(|_| FecError::Backend("gf8 reconstruct"))?;
         collect_originals(received, data_count)
@@ -116,8 +126,17 @@ impl ErasureCoder for Gf8Coder {
         for &(j, bytes) in recovery {
             received[data_count + j] = Some(bytes.to_vec());
         }
-        let rs = ReedSolomon::new(data_count, recovery_count)
-            .map_err(|_| FecError::Config("invalid GF(2^8) shard counts"))?;
+        // Cache the codec by (k, m) exactly as `encode_into`/`reconstruct` do (see the note
+        // there) — this path runs per lossy block on the pump thread.
+        let mut guard = self.rs.lock().unwrap_or_else(|p| p.into_inner());
+        let cached =
+            matches!(&*guard, Some((ck, cm, _)) if *ck == data_count && *cm == recovery_count);
+        if !cached {
+            let rs = ReedSolomon::new(data_count, recovery_count)
+                .map_err(|_| FecError::Config("invalid GF(2^8) shard counts"))?;
+            *guard = Some((data_count, recovery_count, rs));
+        }
+        let rs = &guard.as_ref().expect("cache populated above").2;
         rs.reconstruct_data(&mut received)
             .map_err(|_| FecError::Backend("gf8 reconstruct"))?;
         for (i, h) in have.iter().enumerate() {