Add gRPC Search service with Search and SearchStream RPCs

- Proto definitions for SearchRequest/SearchResponse/SearchResult
- SearchService implementing MusicFs trait
- Query validation (empty check, 256 char max, limit cap at 10k)
- Streaming support via tokio mpsc channel

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

musicfs/crates/musicfs-grpc/build.rs

@@ -0,0 +1,4 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tonic_build::compile_protos("proto/musicfs.proto")?;
+    Ok(())
+}