fix(core): drop PendingSubmission's guards in reverse acquisition order

`PendingSubmission` holds a `SnatchGuard` acquired by its caller and a
`CommandIndices` write guard acquired later in `Queue::allocate_submission`,
but declares them in that same order. Struct fields drop in declaration
order, so the older guard was released first.

`--cfg wgpu_validate_locks` enforces stack-ordered release, so this trips
its assertion on the first submission that goes through this path — which
makes the validator unusable on the buffer-mapping path, since
`flush_writes_for_buffer` reaches it.

Swap the two declarations. No runtime behaviour changes without the cfg;
both guards are still released before the struct's other fields.

Author: AdrianEddy

wgpu-core/src/device/queue.rs

@@ -638,8 +638,12 @@ impl WebGpuError for QueueSubmitError {
 /// [`submit`]: `PendingSubmission::submit`
 pub(crate) struct PendingSubmission<'a> {
     queue: &'a Queue,
-    snatch_guard: SnatchGuard<'a>,
+    // Declared before `snatch_guard` so that it is dropped first. `snatch_guard`
+    // is acquired by the caller and `command_index_guard` in
+    // `Queue::allocate_submission`, so releasing in field order would release the
+    // older guard first — which `--cfg wgpu_validate_locks` rejects.
     command_index_guard: RwLockWriteGuard<'a, CommandIndices>,
+    snatch_guard: SnatchGuard<'a>,
     // Command buffers to be executed, along with trackers for the resources they use.
     pub executions: Vec<EncoderInFlight>,
     // Surface textures referenced by command buffers in this submission. These need to be