[WIP][CUDA][UR] Track USM allocation metadata for cross-device operations

[kekaczma]

Problem:
In multi-device contexts, each device has its own primary CUDA context. When USM memory allocated on device A is accessed from a queue on device B, using cuMemcpyAsync fails because the stream belongs to context B but operates on memory from context A.

Root cause:

• urUSMSharedAlloc/urUSMDeviceAlloc allocate memory in device-specific contexts
• urEnqueueUSMMemcpy receives pointers without knowing their origin device
• Cross-context operations require explicit cuMemcpyPeerAsync with both contexts

Solution:
Track allocation metadata in ur_context to record which device allocated each USM pointer. In urEnqueueUSMMemcpy, query this metadata to detect cross-device copies and use cuMemcpyPeerAsync with explicit source and destination contexts.

Changes:

• Add AllocationMetadata map to ur_context_handle_t with thread-safe access
• Register allocations in urUSMDeviceAlloc and urUSMSharedAlloc
• Unregister in urUSMFree
• Query metadata in urEnqueueUSMMemcpy to detect cross-device copies
• Use cuMemcpyPeerAsync for cross-device, cuMemcpyAsync otherwise

This is a clean, O(1) solution that correctly handles cross-context operations without trial-and-error approaches.