feat: add voxcpm validate CLI for pre-flight training data checks

Add a new `validate` subcommand that checks JSONL training manifests
before starting expensive fine-tuning jobs. This catches format issues,
missing audio files, and data quality problems early.

The validator performs:
- JSONL format validation (each line must be valid JSON)
- Required column checks (text, audio)
- Audio file existence and readability verification
- Duration and text length statistics (min, max, mean, median)
- Optional ref_audio column validation
- Warnings for very short (<0.3s) or very long (>30s) audio samples

Usage:
  voxcpm validate --manifest train.jsonl
  voxcpm validate --manifest train.jsonl --sample-rate 16000 --verbose

The module uses lazy imports for soundfile, so it works even in
minimal environments. Includes 11 unit tests covering all validation
paths.

src/voxcpm/training/__init__.py

@@ -15,6 +15,7 @@ from .data import (
     BatchProcessor,
 )
 from .state import TrainingState
+from .validate import validate_manifest, ValidationResult
 
 __all__ = [
     "Accelerator",
@@ -24,4 +25,6 @@ __all__ = [
     "TrainingState",
     "load_audio_text_datasets",
     "build_dataloader",
+    "validate_manifest",
+    "ValidationResult",
 ]