deps: bump kernels pin to the diag_bending/diag_all corner cross-term fix

[balbasty]

What

Moves the kernels gitlink from be7be08 to 1df9fd3 (current fastfields-kernelsmain). Gitlink-only — git diff-tree between main and this branch reports exactly one modified path, kernels.

What it picks up

kernels PR	commit	why it matters here
#81	1df9fd3	correctness. Corrects the doubled fx1*fy0 / dropped fx0*fy1 corner term in the boundary-corrected diagonal of diag_bending (field 2d/3d, flow 2d/3d) and diag_all (flow 2d/3d). Corrupted field_diag/flow_diag at boundary voxels under sign-flipping bounds with bending active, and any Jacobi preconditioner built from it.
#79	93a954b	restrict.h now includes batch.h for index2sub. The old pin predated it, so this tree could not be compiled by g++ at all.

be7be08 is an ancestor of 1df9fd3, so this is a fast-forward of the pin with no history surprises.

Pin hygiene

1df9fd3 is the squash-merge commit that actually exists on fastfields-kernelsmain, not the pre-merge branch head (81cd9a4, which ceased to exist as a reachable ref when #81 was squashed). Verified it resolves before opening this PR.

Verification

Full CPU suite against this exact kernels commit through the dev-tree symlink chain (cpu-lib/impl -> cpu-impl -> kernels@1df9fd3), make -j4 test CXX=clang++ — 13/13 suites pass, 0 failures:

test_distance 2352 test_distance_mesh 4622
test_distance_spline 704 test_posdef 4012
test_pushpull 308 test_pushpull_backward 6381
test_reg_field 18284 test_reg_flow 11415
test_reg_op 186 test_resize 630
test_restrict 65 test_splinc 4577
test_solve_field (pass)

The regression test that actually fails without kernels#81 lands separately on fastfields-cpu-lib, once its impl pin reaches this commit.

🤖 Generated with Claude Code

https://claude.ai/code/session_016AjQcY78NgbagPSbPJRr6Z

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Generated by Claude Code

[github-actions]

Submodule staleness (non-blocking -- fastfields-lib#15)

submodule	pinned commit	tracks	status	behind by
kernels	1df9fd3	fastfields/fastfields-kernels@main	up to date	0