vfs/disk: Fix EvictDiskVisibilityAndRecreateSafety flake

New() launches background calculateSizeAndPopulateIndex which scans
disk and calls insertBatch. Create does not wait on initDone, so a
file can be written and indexed, discovered by the scan, then removed
from d.info and disk by EvictLRU/EvictBySize. insertBatch then
re-inserted the stale discoveredFile without checking the path still
existed, so Stat succeeded from the index while os.Stat failed.

Re-stat under the lock and skip gone files so evicted keys are not
resurrected.

Fixes #18.

vfs/disk/disk.go

@@ -248,15 +248,25 @@ func (d *DiskFS) calculateSizeAndPopulateIndex() {
 
 // insertBatch populates info/LRU under lock for a bounded batch (follows maxEvictBatch pattern for short critical sections).
 // Size is incremented here only for files actually added (prevents double-count vs. concurrent Create during window).
+// Fail-closed: re-stat each path under d.mu and skip if the file is gone. Create does not wait on
+// initDone, so a file the scanner observed can be Evict/Delete'd (info + os.Remove) before this
+// insert runs. Inserting without a live-file check would resurrect the key in d.info and make
+// Stat succeed while os.Stat fails.
 func (d *DiskFS) insertBatch(batch []discoveredFile) {
 	d.mu.Lock()
 	for _, df := range batch {
-		if _, exists := d.info[df.key]; !exists {
-			fi := vfs.NewFileInfoFromOS(df.osInfo, df.key)
-			d.info[df.key] = fi
-			d.LRU.Add(df.key, fi)
-			d.size += df.size
+		if _, exists := d.info[df.key]; exists {
+			continue
 		}
+		path := d.pathForKey(df.key)
+		st, err := os.Stat(path)
+		if err != nil {
+			continue
+		}
+		fi := vfs.NewFileInfoFromOS(st, df.key)
+		d.info[df.key] = fi
+		d.LRU.Add(df.key, fi)
+		d.size += st.Size()
 	}
 	d.mu.Unlock()
 }