fix: cap crossfade slider max to 4s and clamp in _build_segments

Crossfade duration max was 8s — equal to MMAudio's window (8.0s) and
nearly equal to TARO's (8.192s), which would produce step_s <= 0 and
degenerate segments. With cross-model regen the crossfade must be safe
for the smallest window. Cap sliders to 4s and add a safety clamp in
_build_segments (crossfade <= half window) to guard against stale metadata.

app.py

@@ -279,6 +279,8 @@ def mux_video_audio(silent_video: str, audio_path: str, output_path: str) -> Non
 def _build_segments(total_dur_s: float, window_s: float, crossfade_s: float) -> list[tuple[float, float]]:
     """Return list of (start, end) pairs covering *total_dur_s* with a sliding
     window of *window_s* and *crossfade_s* overlap between consecutive segments."""
+    # Safety: clamp crossfade to < half the window so step_s stays positive
+    crossfade_s = min(crossfade_s, window_s * 0.5)
     if total_dur_s <= window_s:
         return [(0.0, total_dur_s)]
     step_s = window_s - crossfade_s
@@ -2399,7 +2401,7 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     taro_cfg     = gr.Slider(label="CFG Scale", minimum=1, maximum=15, value=8.0, step=0.5, elem_id="taro_cfg")
                     taro_steps   = gr.Slider(label="Sampling Steps", minimum=10, maximum=50, value=25, step=1, elem_id="taro_steps")
                     taro_mode    = gr.Radio(label="Sampling Mode", choices=["sde", "ode"], value="sde", elem_id="taro_mode")
-                    taro_cf_dur  = gr.Slider(label="Crossfade Duration (s)", minimum=0, maximum=8, value=2, step=0.1, elem_id="taro_cf_dur")
+                    taro_cf_dur  = gr.Slider(label="Crossfade Duration (s)", minimum=0, maximum=4, value=2, step=0.1, elem_id="taro_cf_dur")
                     taro_cf_db   = gr.Textbox(label="Crossfade Boost (dB)", value="3", elem_id="taro_cf_db")
                     taro_samples    = gr.Slider(label="Generations", minimum=1, maximum=MAX_SLOTS, value=1, step=1)
                     taro_btn        = gr.Button("Generate", variant="primary")
@@ -2465,7 +2467,7 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     mma_seed     = gr.Number(label="Seed (-1 = random)", value=-1, precision=0, elem_id="mma_seed")
                     mma_cfg      = gr.Slider(label="CFG Strength", minimum=1, maximum=10, value=4.5, step=0.5, elem_id="mma_cfg")
                     mma_steps    = gr.Slider(label="Steps", minimum=10, maximum=50, value=25, step=1, elem_id="mma_steps")
-                    mma_cf_dur   = gr.Slider(label="Crossfade Duration (s)", minimum=0, maximum=8, value=2, step=0.1, elem_id="mma_cf_dur")
+                    mma_cf_dur   = gr.Slider(label="Crossfade Duration (s)", minimum=0, maximum=4, value=2, step=0.1, elem_id="mma_cf_dur")
                     mma_cf_db    = gr.Textbox(label="Crossfade Boost (dB)", value="3", elem_id="mma_cf_db")
                     mma_samples     = gr.Slider(label="Generations", minimum=1, maximum=MAX_SLOTS, value=1, step=1)
                     mma_btn         = gr.Button("Generate", variant="primary")
@@ -2520,7 +2522,7 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     hf_guidance = gr.Slider(label="Guidance Scale", minimum=1, maximum=10, value=4.5, step=0.5, elem_id="hf_guidance")
                     hf_steps    = gr.Slider(label="Steps", minimum=10, maximum=100, value=50, step=5, elem_id="hf_steps")
                     hf_size     = gr.Radio(label="Model Size", choices=["xl", "xxl"], value="xxl", elem_id="hf_size")
-                    hf_cf_dur   = gr.Slider(label="Crossfade Duration (s)", minimum=0, maximum=8, value=2, step=0.1, elem_id="hf_cf_dur")
+                    hf_cf_dur   = gr.Slider(label="Crossfade Duration (s)", minimum=0, maximum=4, value=2, step=0.1, elem_id="hf_cf_dur")
                     hf_cf_db    = gr.Textbox(label="Crossfade Boost (dB)", value="3", elem_id="hf_cf_db")
                     hf_samples      = gr.Slider(label="Generations", minimum=1, maximum=MAX_SLOTS, value=1, step=1)
                     hf_btn          = gr.Button("Generate", variant="primary")