feat(cuinterpose): VMM ownership and allocation lifecycle

[galletas1712]

Summary

Refs #295 (approved).

Track supported VMM allocations through application-visible virtual allocation handles, along with mappings, access permissions, ownership, and sharing state. Implement capture and restore operations that leave never-shared allocations on the native CUDA path.

Review notes

• The shared virtual-allocation-handle namespace is tagged so cuinterpose can distinguish virtual handles from native CUDA handles.
• State owns the mapping from each virtual allocation handle to an allocation identity; callers receive native handles only after resolution inside the shim.
• Virtual multicast handles use this same namespace as a subtype when multicast support is added in feat(cuinterpose): multicast tracking and reconstruction #331.
• cuMemExportToShareableHandle returns a virtual shareable handle. Import resolves its allocation reference and requests the real CUDA export FD from its creator.
• SaveAllocations and LoadAllocations call the host carriers supplied by feat(cuinterpose): host-carrier storage for shared allocation bytes #330. Never-shared allocations remain native CUDA state.

This PR does not add PageBroker storage, backend selection, or export-property promotion.

Validation

On the assembled stack, make -C agent/cmd/cuinterpose test passes formatting, strict Clippy, GNU/musl builds, ABI and artifact checks, Rust tests, and packaged fake-driver lifecycle tests. Physical-GPU tests were not rerun for this terminology-only change.

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