feat: add checkpoint resume and fix train_loss tracking

- Add --resume CLI arg to resume training from a checkpoint
- Restore model, optimizer, scheduler state; continue from saved epoch+1
- Preserve global_step and best_val_loss across resume
- Save run_id in checkpoints for TensorBoard log continuity
- Use logs/run_<timestamp>/ subdirectories to isolate experiment logs
- Fix: replace train_loss in checkpoint dict with global_step to avoid
  KeyError when loading; track global_step through train_one_epoch
- Fix: use global_step (not batch_idx) as TensorBoard x-axis for batch loss
- Fix: print average loss at end of each epoch

Generated by Mistral Vibe (ds-v4-flash).
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/velocity_prediction/config.py

@@ -10,9 +10,10 @@ from pathlib import Path
 
 DATASET_ROOT = Path(__file__).resolve().parents[2] / "dataset"
 
-# Velocity normalization stats (computed from training set)
-VELOCITY_MEAN = [0.859184, -0.783945]   # [vx, vy]
-VELOCITY_STD  = [2.244513,  1.088335]   # [vx, vy]
+# Velocity normalization stats (computed from forward scenes only, 28363 frames)
+# Yaw-compensated horizontal velocity: [v_right, v_forward]
+VELOCITY_MEAN = [-2.902497, 3.837231]   # [v_right, v_forward]
+VELOCITY_STD  = [3.453774,  3.722085]   # [v_right, v_forward]
 
 # TRAIN_SCENES = [
 #     "indoor_forward_3", "indoor_forward_5", "indoor_forward_6",
@@ -38,10 +39,10 @@ VAL_SCENES = [
     # "indoor_forward_3", "indoor_forward_9", "indoor_forward_10",  # Easy
 ]
 TEST_SCENES = [
-    "indoor_forward_7",      # Hard 室内
-    "outdoor_forward_1",     # Easy 室外
-    "outdoor_forward_5"      # Hard 室外
-    # "indoor_forward_3", "indoor_forward_9", "indoor_forward_10",  # Easy
+    # "indoor_forward_7",      # Hard 室内
+    # "outdoor_forward_1",     # Easy 室外
+    # "outdoor_forward_5"      # Hard 室外
+    "indoor_forward_3", "indoor_forward_9", "indoor_forward_10",  # Easy
 ]
 
 
@@ -75,7 +76,7 @@ class GRUConfig:
 class HeadConfig:
     input_dim: int = 128        # GRU hidden_size
     hidden_dim: int = 64
-    output_dim: int = 2         # [vx_body, vy_body]
+    output_dim: int = 2         # [v_right, v_forward]
 
 
 @dataclass