Create directory children through opaque directory grants #409

Scope

Implement FileSystemDirectoryHandle.getDirectoryHandle(name, {create:true}) over a typed opaque get-or-create directory-child authority.

Required browser behavior:

• create and return an empty directory when the named child is absent;
• return an existing directory without removing or changing its contents;
• reject a file collision with TypeMismatchError;
• preserve existing WebIDL option conversion and invalid single-component TypeError behavior;
• map cancellation, denial, stale generation, capacity, and I/O outcomes to browser errors;
• bind each derived handle to the requesting realm/origin/profile/partition and release the native grant exactly once.

Product dependency

Unchanged VS Code calls this operation from htmlFileSystemProvider.mkdir() and from recursive browser folder export in fileImportExport.ts. This is the smallest remaining directory mutation that unlocks a direct Code OSS path.

Acceptance

• pinned WPT and Chrome oracle for create, existing-content preservation, collision, Unicode and invalid names;
• native ABI/export/C11 and AppScene adapter parity;
• origin/profile/partition security, cancellation, stale-generation and navigation retirement;
• 10,000 bounded broker operations and 100 lifecycle cycles with heap/RSS/FD bounds;
• unchanged Code OSS call-site evidence for mkdir and recursive folder export.

Blocked by the missing native opaque directory-child creation authority tracked in SceneTech/AppScene#168. This issue excludes remove/delete, recursive deletion, move/rename, CSS, Window.find, packaging, and AppScene implementation.

Parent: #248.