frost(sdpa): SM107 fp8 sibling missing the #585 LPT_L2 scheduler port — all causal fp8 declines on Rubin

[vedaanta]

Symptom

On cc10.7 at develop tip, every causal/masked per-tensor FP8 SDPA graph fails plan build:

NotImplementedError: sdpa_fwd_prefill_sm100_d128_fp8: SCHED_LPT_L2 decode requires qh_per_kh and seqlen_kv at every call site

test_sdpa_fwd_fp8_sm100.py -m 'L0 or L1': 29 failed / 4 passed on a Rubin board (the 4 survivors are the nomask cases). The same suite is 33/33 on SM100.

Cause

#585 added the LPT_L2 tile scheduler to prefill_d128_fp8_sm100.py (and the adapter's auto causal sched policy now selects it), but the hunk-symmetric port to the SM107 sibling prefill_d128_fp8_sm107.py was not done — the sibling carries only the decode guard, which correctly refuses. 27 vs 7 SCHED_LPT|qh_per_kh references between the two files.

This is the sibling-lockstep hazard (see #579's history): kernel-shared changes must land in both files, and nothing gates it when no Rubin silicon is in the merge loop.

Fix

Port #585's scheduler hunks to the sibling (mechanical, hunk-symmetric), validate causal/BR/SWA fp8 on cc10.7. Interim mitigation if needed: pin sched_policy=SCHED_NATURAL on the Rubin route until the port lands.

[vedaanta]

Status check: the interim mitigation is in place — engines.py's sdpa_fwd_prefill_sm107_fp8 spec pins sched_policies={SCHED_NATURAL} for the Rubin row (comment cites this issue explicitly), so causal/masked fp8 no longer hits the NotImplementedError at plan build; it just runs SCHED_NATURAL instead of LPT_L2.

The mechanical hunk-symmetric port itself also looks done at the kernel level: prefill_d128_fp8_sm107.py and prefill_d128_fp8_sm100.py now have byte-identical SCHED_LPT/qh_per_kh-related code (29/29 references, zero diff) — no more 27-vs-7 gap.

What's still outstanding: the engine capability gate has not been flipped to advertise SCHED_LPT/SCHED_LPT_L2 for the SM107 row, i.e. nothing in-tree confirms causal/BR/SWA fp8 was validated with LPT_L2 on actual cc10.7 silicon yet. Leaving open to track flipping that gate once Rubin validation lands.