[Lane 4 · kotlin-core] SDPA backward — gradient check test for SDPA #1387

Sub-issue of #1384 ([Feature]: scaledDotProductAttention backward — gradients currently stop at attention).

Lane: 4 · Ground-truth/CI
Skill needed: Kotlin tests.
Size: s (a few hours)
Blocked by: nothing

What to do

1. Add a test in skainet-compile/skainet-compile-dag/src/commonTest comparing the tape's gradients against central finite differences (and/or PyTorch values from Lane 1) for causal and non-causal cases, with one padded key.

Acceptance

• Test fails on the current stub and passes with Lane 2
• Result reported back on the parent issue

Notes

You can land this before Lane 2 with @Ignore to make the stub's failure visible.

Coverage matrix: docs/modules/ROOT/pages/reference/ops-status-matrix.adoc · How lanes work: Issue taxonomy