Phase 1 of the HEALTH_DIAG GPU port (Phase 0 inventory landed in
333ea7184). Lands the typed snapshot struct and the mapped-pinned
allocation that subsequent phases write into and read from — no kernel
and no caller wiring yet, deliberately so the type can stabilise before
Phase 2 invests in CUDA shmem reductions against its byte layout.
`HealthDiagSnapshot` (#[repr(C)] POD, 147×4 = 588 bytes) holds every
numeric field emitted across the 7 HEALTH_DIAG log sites. All fields
are `f32` or `u32` so CPU and GPU agree byte-for-byte with no padding
to reason about. Controller fire booleans (`fire_lr`/`fire_tau`/etc.)
are promoted from u8 → u32 per the Phase 0 design review's open-
question #3 — a `[u8; 6]` followed by `f32` would risk implementation-
defined padding mismatch between Rust's #[repr(C)] and CUDA's struct
layout rules; the +24 bytes of cost is acceptable.
`MappedHealthDiagSnapshot` wraps `cuMemHostAlloc(DEVICEMAP|PORTABLE)`
of `sizeof(HealthDiagSnapshot)` plus `cuMemHostGetDevicePointer_v2` —
mirrors the existing `MappedF32Buffer` allocator pattern but typed.
The kernel chain in Phase 2 will write through `dev_ptr()`; the host
emit code in Phase 4 reads through `host_ref()`. No `memcpy_dtoh`,
no `Vec` allocator on the path — consistent with
`feedback_no_htod_htoh_only_mapped_pinned.md`.
Three unit tests pin the layout (`snapshot_size_is_stable`,
`alignment_is_4_bytes`, `default_is_zeroed`) so any future field
add/reorder requires an explicit test update — guards the kernel-side
offset table from silent drift between commits. The size assertion
records the field count breakdown line by line in a comment so the
next maintainer can audit the math without re-deriving it.
Touched: cuda_pipeline/health_diag.rs (+339 LOC new),
cuda_pipeline/mod.rs (+8 re-exports). cargo check clean at 12 warnings
(workspace baseline). Three new unit tests pass on local CPU host (no
GPU required). Audit doc note added per Invariant 7 — explicitly notes
producer kernel + caller migrations land in Phase 2 / 4 commits.
No fingerprint change — separate mapped-pinned alloc, not part of the
param-tensor layout the fingerprint guards.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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