vulkan: portable CPU-proxy fallback for cross-driver -sm tensor AllReduce

The GPU-side cross-device ordering imports each peer's OPAQUE_FD timeline
semaphore, but OPAQUE_FD payloads are driver-private, so the import only works
when all devices share a driver (e.g. two NVIDIA GPUs). On mixed drivers or
vendors it is out of spec.

Add a portable fallback: a helper thread polls each peer's progress/upload
timeline and host-signals a local timeline that the consumer's download is
parked on (core timeline semaphores plus host signal/wait, no imported handle).
Both the chunked pipeline (prefill) and the single-shot (decode) paths are
bridged, so proxy mode no longer drops decode to the meta-backend butterfly.

A capability gate (vkGetPhysicalDeviceExternalSemaphoreProperties plus a
driverUUID match) selects the proxy deterministically on unsupported configs;
GGML_VK_COMM_PROXY forces it, and the import try/catch stays as a safety net.
Measured within ~4% of the native-import path on decode and on par for
prefill, with byte-identical output.

Assisted-by: Claude Opus 4.8

Author: pwilkin