Support no-copy mobile model loading from user-granted files

[leehack]

Problem

The shared model cache API documents Android user-selected model libraries and iOS user-picked files/App Group paths, but the current native loading path is still file-path based. When apps receive an Android Storage Access Framework content URI, or a user-picked iOS security-scoped file that cannot be used as a stable local path, they may need to copy the model into app-private cache before loading.

Desired outcome

Support no-copy model loading where the platform and backend make it feasible:

• Android: file-descriptor/content-URI backed loading for user-granted model files.
• iOS/macOS: security-scoped/user-picked file handling where applicable.
• Keep copy-to-app-cache as the documented fallback when direct loading is unavailable.

Acceptance criteria

• Public API shape clearly distinguishes path-backed files from descriptor/URI-backed handles.
• Unsupported backends fail loudly with actionable diagnostics.
• Tests cover mocked Android URI grants or descriptor-style handles where feasible.
• Docs explain when apps can share an external model library without duplicating the model file.

Context

Follow-up from PR #242, which adds platform-aware model cache strategy constructors and documents explicit mobile sharing flows.