[wieslawsoltes]

Problem

AppScene can now construct Skia Graphite on Dawn's exact Vulkan device, but Vulkan queue operations require external synchronization. Dawn and Graphite would otherwise submit/present concurrently through the same raw VkQueue.

Change

• add an additive v3 exact-device ABI with balanced queue-access acquire/release tokens
• hold Dawn's existing DeviceGuard, the same recursive device mutex used by its Vulkan submit and present paths
• retain both public WGPUDevice and internal DeviceBase lifetimes for the token
• validate the live registry before private dereference and reject foreign, stale, lost, closing, nested, and wrong-thread use
• provide a lexical public C++ wrapper while preserving v1/v2
• wire exports, installed SDK macros, cache/hash inputs, source contracts, and validation docs

Closes #651.

Validation

git diff --check passed. No builds or tests were run under the current fast integration policy.