feat(lora): add preset registry with hf-download helper

lora_stack.py

@@ -120,3 +120,32 @@ def sniff(path: Path | str) -> LoRAInfo:
         diagnostic=diagnostic,
         file_size=file_size,
     )
+
+
+_PRESETS_PATH = Path(__file__).resolve().parent / "presets" / "manifest.json"
+
+
+def load_presets() -> list[dict]:
+    """Load the bundled LoRA preset manifest."""
+    return json.loads(_PRESETS_PATH.read_text())
+
+
+def download_preset(name: str) -> Path:
+    """Download a preset LoRA from HF if not already cached.
+
+    Returns the local path on success. Raises LoRAValidationError if the
+    preset name is unknown OR the HF download fails (network, 404, etc.).
+    """
+    from huggingface_hub import hf_hub_download
+    from huggingface_hub.utils import HfHubHTTPError
+
+    for p in load_presets():
+        if p["name"] == name:
+            try:
+                local = hf_hub_download(repo_id=p["hf_id"], filename=p["filename"])
+                return Path(local)
+            except HfHubHTTPError as e:
+                raise LoRAValidationError(
+                    f"Could not download preset {name!r} from {p['hf_id']!r}: {e}"
+                ) from e
+    raise LoRAValidationError(f"Unknown preset: {name}")

presets/manifest.json

@@ -0,0 +1,30 @@
+[
+  {
+    "name": "RapMachine",
+    "hf_id": "ACE-Step/ACE-Step-v1-RapMachine-LoRA",
+    "filename": "RapMachine.safetensors",
+    "kind": "genre",
+    "default_scale": 0.85
+  },
+  {
+    "name": "Chinese Rap",
+    "hf_id": "ACE-Step/ACE-Step-v1-Chinese-Rap-LoRA",
+    "filename": "ChineseRap.safetensors",
+    "kind": "genre",
+    "default_scale": 0.85
+  },
+  {
+    "name": "Lyric2Vocal",
+    "hf_id": "ACE-Step/ACE-Step-v1-Lyric2Vocal-LoRA",
+    "filename": "Lyric2Vocal.safetensors",
+    "kind": "voice",
+    "default_scale": 0.70
+  },
+  {
+    "name": "Text2Samples",
+    "hf_id": "ACE-Step/ACE-Step-v1-Text2Samples-LoRA",
+    "filename": "Text2Samples.safetensors",
+    "kind": "instrumental",
+    "default_scale": 0.80
+  }
+]

tests/test_lora_presets.py

@@ -0,0 +1,22 @@
+"""L1 tests for the LoRA preset registry."""
+
+from __future__ import annotations
+
+import lora_stack as ls
+
+
+def test_load_presets_returns_four():
+    presets = ls.load_presets()
+    assert len(presets) == 4
+    names = [p["name"] for p in presets]
+    assert "RapMachine" in names
+    assert "Lyric2Vocal" in names
+
+
+def test_preset_has_required_fields():
+    presets = ls.load_presets()
+    for p in presets:
+        assert "name" in p
+        assert "hf_id" in p
+        assert "default_scale" in p
+        assert 0 <= p["default_scale"] <= 1.5