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

[balbasty]

What

Moves the impl gitlink from 41d66b3 to 5e2c78e (current fastfields-cpu-implmain). Gitlink-only — git diff-tree between main and this branch reports exactly one modified path, impl.

Resulting chain

cpu-lib (this PR) -> cpu-impl 5e2c78e -> kernels 1df9fd3

which brings the CPU path onto:

kernels PR	commit	effect
#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). The defect corrupted field_diag/flow_diag at boundary voxels under sign-flipping bounds (Zero/DST1/DST2) with bending active, and hence any Jacobi preconditioner built from that diagonal.
#79	93a954b	restrict.h includes batch.h for index2sub; the previous chain predated it, so g++ could not compile this tree at all.

Pin hygiene

Both pins are the squash-merge commits that actually exist on their repos' main, not pre-merge branch heads — this project has hit dangling pins that way before. Verified each resolves before opening.

Verification

Full CPU suite over this exact chain, 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)

Note the existing suite is green both before and after this bump — nothing currently on main covers the corrected boundary corners. The regression test that does fail without kernels#81 follows immediately in a separate PR, which this bump unblocks.

🤖 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
impl	5e2c78e	fastfields/fastfields-cpu-impl@main	up to date	0