(docs): add sprint log entries for pre-flight and registry unification.

docs/sprint_log.md

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+# Sprint Log
+
+## @model-expansion-preflight-2025-08-21
+**Goal:** Reinforce training script contracts and registry hook without behavior changes.
+**Changes:**
+- Reproducibility seeds (python/numpy/torch/cuda).
+- Optional cuDNN deterministic settings.
+-Typo fix: "Reseample" -> "Resample".
+- Diagnostics fix: per-fold accuracy logs use correct variable.
+- Explicit dtypes in TensorDataset (float32/long).
+**Tests:**
+- Preprocess: ✅
+- Train (figure2, 1 epoch): ✅
+- Inference smoke: ✅
+**Notes:** Baseline intact; high CV variance due to class imbalance recorded for later migration.
+
+## @model-expansion-registry-2025-08-21
+**Goal:** Make model lookup a single source of truth and expose dynamic choices for CLI/infra.
+**Changes:** 
+- Added `models/registry.py` with `choices()` and `build()` helpers.
+- `scripts/train_model.py` imports registry, uses `choices()` for argparse and `build()` for contruction.
+- Removed direct model selection logic from training script. 
+**Tests:**
+- Train (figure2) via registry: ✅
+- Inference unchanged paths: ✅
+**Notes:** Artifacts remain `outputs/{model}_model.pth` to avoid breaking validator; inference arch flag to be added next.