revert(chatterbox-fast): drop context-priming (§1.6) — discard-cut leaks context

Revert the priming feature from d707439. Live A/B caught an audible artifact: the
context-priming discard-cut left part of the throwaway prefix in the output, so a
clause ("...without a trace of sarcasm,") was spoken an extra time.

Root cause is structural: generate() returns one finished waveform with no marker
for where the prefix ends, and the model renders the same prefix with different
timing when followed by content than when generated solo — so the duration-estimate
+ energy-minimum cut is a guess and can leave a sliver (or a whole clause) of prefix
in. A reliable cut would need token-level access (the abandoned native-streaming
arc) or a per-chunk ASR/alignment pass (heavy, still imperfect, eats the latency
budget). Fails the agreed bar: "keep only if it closes the gap without a seam."

Kept from d707439: the .gitignore (build artifacts). NOT re-applied: the bundled
margin_first fix — wiring it would shrink chunk 1 (more joins = worse coherence),
against the operator's priority, and margin=0.8 there is already starvation-safe.

Coherence loss at joins stays an accepted limitation; cold streaming was judged
"really good". Phase 1 + Phase 2 parity/perf untouched. Next: Phase 3 deploy.

stacks/chatterbox-fast/test_scheduler.py

@@ -45,19 +45,13 @@ class FakeClock:
 def make_generator(clock: FakeClock, *, true_rtf: float, sec_per_char: float):
     """A fake generate() that costs realistic wall-clock and returns audio_sec.
 
-    Audio duration is proportional to content length; generation costs
-    ``audio_sec / true_rtf`` of (simulated) wall-clock, advancing the clock. A
-    primed chunk (context given) costs extra: a context-solo pass plus the
-    context portion of the joint pass — modelling the ~2× cost the scheduler
-    must budget for.
+    Audio duration is proportional to text length; generation costs
+    ``audio_sec / true_rtf`` of (simulated) wall-clock, advancing the clock.
     """
 
-    def generate(text: str, context: str | None):
-        audio_sec = len(text) * sec_per_char  # content only (context discarded)
-        gen_audio = audio_sec
-        if context:
-            gen_audio += 2 * len(context) * sec_per_char  # ctx-solo + ctx in joint
-        clock.t += gen_audio / true_rtf
+    def generate(text: str):
+        audio_sec = len(text) * sec_per_char
+        clock.t += audio_sec / true_rtf
         return None, audio_sec
 
     return generate
@@ -191,43 +185,6 @@ def test_full_text_reconstructed():
     assert joined == " ".join(split_sentences(PARAGRAPH))
 
 
-def test_priming_fires_but_never_on_chunk_zero():
-    """With prime_first_n=2, priming is best-effort (affordability-gated): it fires
-    on at least one early join at fleet RTF, and NEVER on chunk 0 (latency-critical)."""
-    cfg = ChunkConfig(prime_first_n=2)
-    results = run(true_rtf=3.8, cfg=cfg)
-    assert results[0].primed is False
-    assert any(r.primed for r in results[1:]), "expected at least one primed join"
-    # Priming only ever lands on chunks 1..N.
-    assert all(not r.primed for r in results if r.index > cfg.prime_first_n)
-
-
-def test_priming_never_starves_at_fleet_rtf():
-    """Priming must keep the no-starvation guarantee for any GPU at/above the
-    rtf_prior floor (3090 ~3.4, A6000 ~3.8–4.0)."""
-    cfg = ChunkConfig(prime_first_n=2)
-    for rtf in (4.0, 3.8, 3.4):
-        results = run(true_rtf=rtf, cfg=cfg)
-        assert all(not r.starved for r in results), (
-            rtf, [(r.index, r.drained) for r in results if r.starved]
-        )
-
-
-def test_priming_self_skips_on_slow_gpu_no_starvation():
-    """On a slower-than-fleet GPU (RTF below the prior), priming gracefully
-    self-skips rather than starving — the guarantee holds, priming just stops."""
-    cfg = ChunkConfig(prime_first_n=2)
-    results = run(true_rtf=3.0, cfg=cfg)
-    assert all(not r.starved for r in results)
-    assert not any(r.primed for r in results)  # degraded to cold
-
-
-def test_priming_off_by_default():
-    """Default config primes nothing (so context is never requested)."""
-    results = run(true_rtf=3.8)
-    assert all(not r.primed for r in results)
-
-
 def _main():
     results = run(true_rtf=3.8)
     print(f"{'idx':>3} {'chars':>5} {'audio_s':>8} {'gen_s':>7} "