Op provider: route runtime selection by full device

Row: BACKEND-ACCEL-PROVIDER

The backend registry is keyed by Device{type,index}, but the operation provider resolves runtime state by DeviceType. Every queue-bearing wrapper discards the index before GetOp.

This is memory-unsafe for the portable CPU reference tier. If index 0 is host-addressable, it can install and cache vt-cpu-ref for the whole type. A discrete index can then select that host function over non-host-addressable memory. Fallback draining also calls backend index 0 rather than the queue's backend.

The same runtime path has a publication race. MaybeInstallReferenceTier appends a provider after main() starts without synchronization. Two concurrent first misses can race slot.count and slot.providers.

Fresh review of #2377 found the device-index defect. Independent adversarial analysis confirmed that synchronizing every GPU does not fix host-addressability. A shared selected pointer with separate relaxed ref_selected state is also insufficient.

Acceptance:

• Provider definitions remain type-keyed and immutable after static registration.
• Runtime selection state, capability state, reference identity, and negative caches are keyed by the full Device.
• Queue-bearing execution APIs carry Queue::device through normal and fallback resolution.
• A CPU reference provider is a synthetic per-device candidate. Runtime code does not append it to the shared provider list.
• Every reference invocation checks the exact backend's host-addressability and drains that exact backend.
• A hardware-free two-index test covers both host-addressability polarities, resolution order, exact drain routing, and an already-warm reference selection.
• A concurrent first-miss test passes under ThreadSanitizer and exposes one coherent selection.
• Native provider ordering, disable behavior, statistics, and the cached native hot path retain their existing contracts.
• Cached decline paths cannot bypass per-invocation reference eligibility or draining.

Start by tracing the BACKEND-ACCEL-PROVIDER path through GetOp, Queue::device, and MaybeInstallReferenceTier, then inspect how fallback draining and cached decline paths resolve providers. Use the acceptance list as the completion criteria, including the hardware-free two-index coverage and the concurrent first-miss test under ThreadSanitizer.