fix issues

docs/02_components/helpers/h04_faiss_index_manager_spec.md

@@ -27,11 +27,25 @@ class IFaissIndexManager(ABC):
     @abstractmethod
     def load_index(self, path: str) -> FaissIndex:
         pass
+    
+    @abstractmethod
+    def is_gpu_available(self) -> bool:
+        pass
+    
+    @abstractmethod
+    def set_device(self, device: str) -> bool:
+        """Set device: 'gpu' or 'cpu'."""
+        pass
 ```
 
 ## Component Description
 
-Manages Faiss indices for AnyLoc retrieval (IVF, HNSW options).
+Manages Faiss indices for DINOv2 descriptor similarity search. H04 builds indexes from UAV image descriptors for:
+1. **Loop closure detection**: Find when UAV revisits previously seen areas within the same flight
+2. **Chunk-to-chunk matching**: Match disconnected chunks to each other
+3. **Flight-to-flight matching**: Match current flight to previous flights in same area
+
+**Index Source**: Descriptors are computed from UAV images using F08's DINOv2 encoder, NOT from satellite images. The index enables finding similar UAV viewpoints.
 
 ## API Methods
 
@@ -76,9 +90,18 @@ Manages Faiss indices for AnyLoc retrieval (IVF, HNSW options).
 
 **External**: faiss-gpu or faiss-cpu
 
+## GPU/CPU Fallback
+
+H04 supports automatic fallback from GPU to CPU:
+- `is_gpu_available()`: Returns True if faiss-gpu is available and CUDA works
+- `set_device("gpu")`: Use GPU acceleration (faster for large indexes)
+- `set_device("cpu")`: Use CPU (fallback when GPU unavailable)
+
 ## Test Cases
 
-1. Build index with 10,000 descriptors → succeeds
-2. Search query → returns top-k matches
+1. Build index with 10,000 UAV image descriptors → succeeds
+2. Search query UAV descriptor → returns top-k similar UAV frames
 3. Save/load index → index restored correctly
+4. GPU unavailable → automatically falls back to CPU
+5. Add descriptors incrementally → index grows correctly