Create file children through opaque directory grants

[wieslawsoltes]

Scope

Implement FileSystemDirectoryHandle.getFileHandle(name, {create:true}) over the typed opaque child authority tracked by SceneTech/AppScene#162.

Browser behavior follows the pinned WPT contract:

• create and return an empty file when the named child is absent;
• return an existing file without truncating it;
• reject a directory collision with TypeMismatchError;
• preserve WebIDL option conversion and existing invalid-name TypeError behavior;
• map denied, stale, cancelled, capacity, and I/O outcomes to browser errors without exposing raw paths or native objects;
• bind every derived handle to the requesting origin/profile/partition and release its native grant exactly once across realm/navigation retirement.

Acceptance

• pinned WPT and Chrome oracle for create, existing-content preservation, collision, invalid-name, and error mapping;
• native adapter/ABI/export/C11 parity with AppScene#162;
• cross-origin/profile/partition rejection plus cancel/stale-generation/retire coverage;
• 10k bounded broker operations and 100 lifecycle cycles with heap/RSS/FD bounds;
• unchanged VS Code browser provider file-create/write path through getFileHandle(...,{create:true}) plus createWritable().

This issue excludes directory creation, removeEntry()/remove(), move/rename, recursive deletion, and package paths tracked by #131/#252.

Parent: #248.

[wieslawsoltes]

Implementation is ready in PR #406 at exact head a5b45266641a747fe7d8af647889a0fbc554454e, based on e98b885d487e3e2e560ebe1917d3cdcdb0dc4c58. Local changed-path gates are green: Chrome 153 WPT reduction 3/3; focused native create/runtime and 100-cycle lifecycle; C++ broker/C11 ABI 2/2; 10k broker 0.029s with +96 KiB RSS and 0 FD; export contract 2/2; all three symbols present; AppScene cac13616 request/status/field adapter assertions pass. The broad local aggregate reaches the separately documented pre-existing writer close assertion only after all new create assertions pass. Direct exact-head hosted gates are queued; NuGet is being canceled as unrelated.

[wieslawsoltes]

Completed by PR #406, merged as ad33c45305f4cabbb87d378686539d39efb606c5. Exact-head Linux, macOS, portable V8, native-document, and NativeAOT 11/12 checks all passed. The merged runtime now implements getFileHandle(name, {create:true}) over the opaque AppScene parent/derived-grant ABI, including browser error mapping, navigation cancellation, exactly-once release, and bounded lifecycle gates. Post-merge CI and NuGet tails were canceled.