feat: add seed support for reproducible generation in v1 and v2

- Exposed 'seed' parameter in VoxCPMModel and VoxCPM2Model generation methods.
- Added PyTorch RNG seed setting before inference runs.
- Handled 'retry_badcase' seed adjustment by incrementing the seed value on retries.
- Exposed 'self.last_successful_seed' as a model attribute for UI integrations.
- Propagated 'seed' parameter to high-level pipeline class and CLI tools (cli.py).
- Added '--seed' flag to full-finetune and LoRA inference scripts.
- Configured validation audio generation in training script to use a fixed seed for objective comparison on TensorBoard.
- Added comprehensive unit tests in CLI test files to validate seed parsing and propagation.
- Updated English and Chinese READMEs with seed usage examples.

README.md

@@ -111,6 +111,7 @@ wav = model.generate(
     text="VoxCPM2 is the current recommended release for realistic multilingual speech synthesis.",
     cfg_value=2.0,
     inference_timesteps=10,
+    seed=42,
 )
 sf.write("demo.wav", wav, model.tts_model.sample_rate)
 print("saved: demo.wav")
@@ -134,6 +135,7 @@ wav = model.generate(
     text="VoxCPM2 is the current recommended release for realistic multilingual speech synthesis.",
     cfg_value=2.0,
     inference_timesteps=10,
+    seed=42,
 )
 sf.write("demo.wav", wav, model.tts_model.sample_rate)
 ```
@@ -147,6 +149,7 @@ wav = model.generate(
     text="(A young woman, gentle and sweet voice)Hello, welcome to VoxCPM2!",
     cfg_value=2.0,
     inference_timesteps=10,
+    seed=42,
 )
 sf.write("voice_design.wav", wav, model.tts_model.sample_rate)
 ```
@@ -167,6 +170,7 @@ wav = model.generate(
     reference_wav_path="path/to/voice.wav",
     cfg_value=2.0,
     inference_timesteps=10,
+    seed=42,
 )
 sf.write("controllable_clone.wav", wav, model.tts_model.sample_rate)
 ```
@@ -213,6 +217,7 @@ voxcpm design \
 voxcpm design \
   --text "VoxCPM2 brings studio-quality multilingual speech synthesis." \
   --control "Young female voice, warm and gentle, slightly smiling" \
+  --seed 42 \
   --output out.wav
 
 # Voice cloning (reference audio)