fix(video): 解决视频时间戳处理和编码参数问题

- 统一归零视频起始时间戳，避免源素材非 0 起始 PTS 造成封装后首帧冻结
- 修改 setpts 滤镜表达式为 setpts=PTS-STARTPTS 格式
- 为所有速度调整场景应用标准化的时间戳处理
- 添加视频编码参数测试文件，确保 B 帧在各种硬件加速下被禁用
- 为软件、QSV 和 CUDA 硬件加速添加 B 帧禁用测试用例

tests/unit/test_render_video_effects.py

@@ -137,7 +137,22 @@ def test_ospeed_does_not_trigger_filter_complex(tmp_path):
     # 验证 setpts 滤镜
     vf_idx = command.index('-vf')
     vf_value = command[vf_idx + 1]
-    assert 'setpts=2.0*PTS' in vf_value
+    assert 'setpts=2.0*(PTS-STARTPTS)' in vf_value
+
+
+def test_default_speed_always_normalizes_pts(tmp_path):
+    handler = _create_handler(tmp_path)
+    render_spec = RenderSpec()
+    output_spec = OutputSpec(width=1080, height=1920, fps=30)
+
+    filters = handler._build_video_filters(
+        render_spec=render_spec,
+        output_spec=output_spec,
+        duration_ms=6000,
+        source_duration_sec=10.0,
+    )
+
+    assert 'setpts=PTS-STARTPTS' in filters
 
 
 def test_ospeed_combined_with_speed(tmp_path):
@@ -153,7 +168,7 @@ def test_ospeed_combined_with_speed(tmp_path):
         source_duration_sec=10.0,
     )
 
-    assert 'setpts=1.5*PTS' in filters
+    assert 'setpts=1.5*(PTS-STARTPTS)' in filters
 
 
 def test_ospeed_with_complex_effects(tmp_path):
@@ -171,7 +186,7 @@ def test_ospeed_with_complex_effects(tmp_path):
     # zoom 触发 filter_complex
     assert '[v_base]' in filters
     # setpts 在 base_chain 中
-    assert 'setpts=2.0*PTS' in filters
+    assert 'setpts=2.0*(PTS-STARTPTS)' in filters
     # zoom 正常处理
     assert "overlay=0:0:enable='between(t,1.0,3.0)'" in filters
 
@@ -188,8 +203,8 @@ def test_only_first_ospeed_is_used(tmp_path):
         source_duration_sec=10.0,
     )
 
-    assert 'setpts=2.0*PTS' in filters
-    assert 'setpts=5.0*PTS' not in filters
+    assert 'setpts=2.0*(PTS-STARTPTS)' in filters
+    assert 'setpts=5.0*(PTS-STARTPTS)' not in filters
 
 
 def test_ospeed_affects_tpad_calculation(tmp_path):