Split file capabilities and clarify scheduler trait names.

[samuel-williams-shopify]

Generic file consumers can now require individual operations, such as FileRead + FileWrite, without requiring a backend to implement the entire file interface. Name the broad interfaces FileScheduler and SocketScheduler to distinguish the scheduler capabilities from their resources.

Introduce FileOperation for the shared associated resource type and split the core file operations into independent traits. FileStat owns the metadata output type; FileAdvice and FileAllocate depend only on the shared base. FileScheduler is an automatically implemented aggregate, with ordinary and _submit forms grouped in their operation trait. Rename the existing Socket trait to SocketScheduler; its operation-level split remains separate follow-up work. Native, Tokio, and simulated backends preserve their existing operation behavior.

This changes the public implementor interface: implement individual file traits instead of File, move fully qualified associated-type paths to FileOperation/FileStat, and import operation traits for method calls on concrete schedulers. Rename broad bounds and imports to FileScheduler/SocketScheduler, and socket implementations to impl SocketScheduler. Update modules, exports, examples, shared contracts, release notes, and migration guidance. Partial backends and compile-time checks exercise the smaller capability requirements. Record SocketOperation as the agreed base name for the separate socket refactor.