…sible

When a buffer on a discrete GPU needs to be accessed from a different
device and P2P access is not enabled, migrate the data through a USM
HOST staging buffer instead of returning UR_RESULT_ERROR_UNSUPPORTED_FEATURE.

The migration uses a two-step copy:
1. Synchronous device->host copy using the source device's own command
   list (the destination device cannot reach source device memory
   without P2P).
2. Async host->device copy enqueued on the caller's command list (host
   memory is accessible by all devices, so this is safe).

Before the device->host copy, any pending operations on the caller's
command list are ordered and drained via zeCommandListAppendWaitOnEvents
+ zeCommandListHostSynchronize, ensuring prior kernel writes to the
source buffer are visible.  A fully synchronous fallback is used when
no command list is available (e.g. urMemGetNativeHandle).

Only one staging buffer is kept alive at a time: it is released at the
start of the next migration after zeCommandListHostSynchronize confirms
the previous async copy has completed.

A new ensureDeviceAlloc helper allocates the destination device buffer
without the activeAllocationDevice side-effect of allocateOnDevice,
so the active-device state is only updated after the async copy is
successfully enqueued.

Fixes: intel#22007
Fixes: intel#22008

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>

Add four conformance tests exercising discrete buffers accessed from
two different device queues when P2P access is not available.

Tests covering the async migration path (cmdList != nullptr, triggered
by urEnqueueMem* operations):
- AsyncFillThenReadOnSecondQueueWithWait: fills a buffer on queues[0]
  and reads it on queues[1] using an explicit event dependency.
- PingPongFillBetweenTwoDeviceQueues: alternates fills between queues[0]
  and queues[1], each read on the opposite queue using event dependencies.
- ChainedAsyncOpsAcrossQueuesWithEvents: chains fill, blocking write,
  and read across two queues using cross-queue events.

Test covering the synchronous fallback path (cmdList == nullptr,
triggered by urMemGetNativeHandle):
- SyncFallbackMigrationViaNativeHandle: fills the buffer on device 0,
  calls urMemGetNativeHandle for device 1 to trigger synchronous
  host-staged migration, then verifies the data on device 1.

All tests add an explicit queues.size() < 2 guard (GTEST_SKIP) in case
the fixture minimum-device requirement changes, and cross-queue ordering
is expressed with events throughout to properly exercise the async
migration path.

A dedicated L0 v2 adapter runner (discrete_buffer_host_migration.cpp)
reuses the conformance test source under UR_LOADER_USE_LEVEL_ZERO_V2.

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>