fix: Update directory mtime when re-listing already-sealed directories

When a directory is re-listed after being sealed (e.g., after external
file additions), its mtime needs to be updated to reflect the latest
child modification time. This fix ensures that for the `inode != parent`
path in `listObjectsSlurp`, when `alreadySealed && hasItems`, we call
`updateDirectoryMtime()` to update the directory's mtime from its
children.

This resolves the TestDirMTime failure where dir2's mtime was not being
updated after newfile was added externally, because the update logic
only existed in the `inode == parent` path.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ovaistariq

core/dir.go

@@ -392,6 +392,11 @@ func (parent *Inode) listObjectsSlurp(inode *Inode, startAfter string, sealEnd b
 				inode.mu.Lock()
 				sealSucceeded = inode.sealDirWithValidation()
 				alreadySealed = !sealSucceeded && inode.dir.listDone
+
+				// Special case: if already sealed but got items, update mtime
+				if alreadySealed && hasItems {
+					inode.updateDirectoryMtime()
+				}
 				inode.mu.Unlock()
 
 				// Reacquire parent lock and validate generation
@@ -410,6 +415,11 @@ func (parent *Inode) listObjectsSlurp(inode *Inode, startAfter string, sealEnd b
 				inode.mu.Lock()
 				sealSucceeded = inode.sealDirWithValidation()
 				alreadySealed = !sealSucceeded && inode.dir.listDone
+
+				// Special case: if already sealed but got items, update mtime
+				if alreadySealed && hasItems {
+					inode.updateDirectoryMtime()
+				}
 				inode.mu.Unlock()
 
 				// Validate parent generation before marking gap