Add optional file_id to FileRemoved event for index cleanup

Oracle fix: FileRemoved now includes Option<FileId> to enable
direct index deletion without metadata lookup race condition

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

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

musicfs/crates/musicfs-core/src/events.rs

@@ -1,4 +1,4 @@
-use crate::types::{OriginId, VirtualPath};
+use crate::types::{FileId, OriginId, VirtualPath};
 use tokio::sync::broadcast;
 
 pub struct EventBus {
@@ -34,6 +34,7 @@ pub enum Event {
     },
     FileRemoved {
         path: VirtualPath,
+        file_id: Option<FileId>,
     },
     FileModified {
         path: VirtualPath,