fix(bridge): WAV header data_size for IEEE float32 (was 16-bit byte count)

- finishWav call sites accumulated n*4 (stereo 16-bit) but float32 writer
  emits n*8 bytes -> RIFF/data chunk sizes half the real value; players and
  soundfile truncated renders to half length
- gate test now asserts file size == 44 + data_size to catch this

native_bridge/src/RenderFxJob.cpp

@@ -881,7 +881,7 @@ int run_render_fx_job(const std::string& jobPath, const std::string& inPath,
     for (uint64_t pos = 0; pos < in.L.size(); pos += chunk) {
         const uint32_t n = (uint32_t)std::min<uint64_t>(chunk, in.L.size() - pos);
         writeFrames(wav, in.L.data() + pos, in.R.data() + pos, n);
-        dataBytes += (uint64_t)n * 4;
+        dataBytes += (uint64_t)n * 8;  // stereo float32: 2ch * 4B
     }
     finishWav(wav, dataBytes);
     wav.close();