maven: embedder config validation + docs (task 1)

- Tests: all-three-set → ok; one-missing → error; nil → ok
- Log message on HashEmbedder fallback in voice.go
- Document floor mode in START.md

Co-Authored-By: opencode <opencode@anthropic.com>

START.md

@@ -58,6 +58,8 @@ Config path: `~/.config/maven/mavend.json`. Full example with all options:
 }
 ```
 
+Omit the `embedder` block entirely to use the deterministic HashEmbedder floor (no ML, no ONNX runtime dependency). Useful for testing or low-resource setups.
+
 ## mavsttd — STT worker (optional, remote whisper.cpp)
 
 Requires `LD_LIBRARY_PATH` to include deps/lib (for libwhisper.so, libggml-vulkan.so).