feat(modules): GroupFilesService + files::shared_collection handle (§11.6 files C3)

- shared/group_files: GroupFilesService trait (mirror FilesService, no
  group id arg — owner resolved from cx.app_id), GroupFilesError (clone of
  FilesError + CollectionNotShared) with From<FilesError>, Noop.
- services: group_files field + with_group_files setter (default Noop).
- group_files_service: GroupFilesServiceImpl — owning_group via the
  registry resolver (kind=files) → CollectionNotShared; reads open
  (script_gate), writes fail closed (script_gate_require_principal);
  reuses validate_files_collection + the NewFile/FileUpdate validators +
  sanitize_stored_content_type + the per-file size cap; no events.
- sdk/files: GroupFilesHandle + files::shared_collection(name) + the
  create/head/get/update/delete/list group methods (Blob in/out).
- picloud: construct FsGroupFilesRepo (sharing the files root + size cap)
  + GroupFilesServiceImpl, .with_group_files(...).

Unit tests: off-chain → CollectionNotShared; anon read OK / anon write
Forbidden; oversize → TooLarge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/shared/src/lib.rs

@@ -17,6 +17,7 @@ pub mod execution_log;
 pub mod files;
 pub mod group;
 pub mod group_docs;
+pub mod group_files;
 pub mod group_kv;
 pub mod http;
 pub mod ids;
@@ -68,6 +69,7 @@ pub use files::{
 };
 pub use group::Group;
 pub use group_docs::{GroupDocsError, GroupDocsService, NoopGroupDocsService};
+pub use group_files::{GroupFilesError, GroupFilesService, NoopGroupFilesService};
 pub use group_kv::{GroupKvError, GroupKvService, NoopGroupKvService};
 pub use http::{HttpError, HttpRequest, HttpResponse, HttpService, NoopHttpService};
 pub use ids::{