[1.28.0-rc] Perf improvements for instruction cache + clean up for FROST LA bprop + improve numerics (#759)

[Anerudhan]

Clean cherry-pick of the #759 squash commit (60ece2a27, merged to develop 2026-08-27) onto 1.28.0-rc. No conflicts; touched FROST linear-attention kernels and test_la.py compile-checked.

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