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/src/lib.rs

@@ -1 +1,12 @@
-#![allow(dead_code)]
+pub mod proto {
+    pub mod musicfs {
+        pub mod v1 {
+            tonic::include_proto!("musicfs.v1");
+        }
+    }
+}
+
+mod search_service;
+
+pub use proto::musicfs::v1::music_fs_server::{MusicFs, MusicFsServer};
+pub use search_service::SearchService;