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/manager-core/src/lib.rs

@@ -54,6 +54,7 @@ pub mod group_collection_repo;
 pub mod group_docs_repo;
 pub mod group_docs_service;
 pub mod group_files_repo;
+pub mod group_files_service;
 pub mod group_kv_repo;
 pub mod group_kv_service;
 pub mod group_members_repo;
@@ -186,6 +187,7 @@ pub use group_collection_repo::{GroupCollectionResolver, PostgresGroupCollection
 pub use group_docs_repo::{GroupDocsRepo, GroupDocsRepoError, PostgresGroupDocsRepo};
 pub use group_docs_service::GroupDocsServiceImpl;
 pub use group_files_repo::{FsGroupFilesRepo, GroupFilesRepo, GroupFilesRepoError};
+pub use group_files_service::GroupFilesServiceImpl;
 pub use group_kv_repo::{GroupKvRepo, GroupKvRepoError, PostgresGroupKvRepo};
 pub use group_kv_service::GroupKvServiceImpl;
 pub use group_members_repo::{