codec: drop the fused streaming tail

The fused mode appended the codec stream tail to the predictor frame
graph, so one compute produced both a frame's codes and its 80 ms of
audio with no host round trip. The experiment applied to max_batch 1
with a streaming synthesis only, it cost throughput against the
buffered flush that stays the default, and it kept a second frame
graph, its ring inputs and an init flag alive for that single case. It
is not worth keeping.

Remove the tail helpers, the fused graph of CodePredGraphSet, the
codec_fused field of qt_init_params, the --codec-fused flag of both
tools and the harness switch that exercised it. The predictor frame
unroll and the in graph sampler are untouched.

tests/debug-customvoice-cossim.py

@@ -181,7 +181,6 @@ def main():
     n = min(aa.size, ab.size)
     print(f"[Cossim] WAV stft_cos: {cc.stft_cos(aa.ravel()[:n], ab.ravel()[:n]):.6f} samples: {n}")
 
-    cc.fused_pass(cmd, text, DUMP_CPP)
 
 if __name__ == "__main__":
     main()