[arcusbuilds]

Summary

Fixes #1578. Chunk-mode MomentumDeltaNet drifted from the recurrent definition once μ ≲ 0.8 (error 0.97 at μ = 0.5, NaN at the μ = e^-2 clamp): the chunk reference built its in-chunk mask by subtracting two cumulative sums, which cancel in fp32. It now builds the mask from within-chunk decay matrices, so every exponent is ≤ 0 and nothing is subtracted. Only the PyTorch reference changes; chunk_momentum_delta_rule runs autograd through it, so training forward and backward are both fixed.

Test plan

• New test_full_momentum_chunk_ref_decaying_gates (μ = 0.8, the 0.135 clamp, per-token gates; checks o, final S/M and all input grads against recurrent_momentum_delta_rule_ref): fails 3/3 on main, passes here.
• tests/ops/test_momentum_delta.py: 18/18 pass, plain and with FLA_CI_ENV=1.
• Other dependent tests from find_dependent_tests.py: test_gdn2.py still running locally; test_dplr_tilelang.py not run locally (tilelang not installed). Neither imports the momentum ops.
• RTX 2000 Ada Laptop (sm_89), torch 2.11.0+cu128, triton 3.6.0.

Benchmark / NCU

chunk_momentum_delta_rule fwd+bwd, B2 T2048 H4 D64 bf16, chunk 64, same GPU, 3 alternating rounds: main 132.6 / 136.7 / 134.5 ms, this PR 133.1 / 139.9 / 131.0 ms. Neutral. No varlen run (the op doesn't support cu_seqlens).

Breaking changes

None. Outputs change only where the old path was wrong. The β + 1e-6 offset is dropped, so β is used exactly, as in the recurrent reference.

Checklist

• I have read CONTRIBUTING.md and follow its conventions (code style, docstrings, commit prefixes).
• I have read AGENTS.md and, where my change matches its scope, the relevant skill under .agents/skills.
• Dependent tests pass locally or in CI, and new behavior is covered by tests where applicable (tick as N/A for changes with no testable code, e.g. docs-only).
• Kernel changes include same-hardware before/after benchmark numbers, dense + varlen where applicable (tick as N/A when no kernel code changed).
• This PR is minor/cosmetic-only (typo, formatting, style-only tweaks) — tick only if it is, and justify below.
• I understand this PR exceeds 500 changed lines and have explained its single purpose and why it should stay together below.