Task 5 of the GPU log ring plan. Adds `gpu_log` module behind the
`cuda-diag-log` feature gate:
- `LogRecord` (#[repr(C)], 64 B) mirroring the C-side layout in
`gpu_log_ring.cu`; const compile-time size assertion catches drift.
- `LogRing::alloc()` allocates and zero-initialises a 32k-slot
mapped-pinned ring (2 MiB pinned).
- `alloc_step_counter()` allocates the device-side i32 step counter.
- Decoder dispatch table with per-(kid, rt) decoders for
`KID_SMOOTHNESS_CONTROLLER` × {RT_INPUT, RT_STATE, RT_OUTPUT}
emitting structured tracing::info! events.
- `spawn_drain_task()` polls a mapped-pinned step-counter shadow at
500 ms cadence, walks new slots, validates magic + step, dispatches
to decoders. Emits tracing::warn! when the drainer falls behind the
ring window (no silent record drops).
Kernel ID + record-type constants are a manual mirror of
`crates/ml-alpha/cuda/gpu_log_ids.h`; drift is caught by the const
size assertion at compile time and (subsequently) by build.rs.
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