fix(ad): Enzyme rule on single-grid apply_bc! segfaults on Linux x86_64 #40

A custom Enzyme rule on the single-grid apply_bc!(::AbstractArray, ::AbstractGrid) segfaulted on Linux x86_64 (Julia 1.10 threads-1/2 and 1.12 threads-1) while passing cleanly on macOS/aarch64 across every version tried.

[2125] signal (11.128): Segmentation fault
in expression starting at test/autodiff.jl:19
apply_bc! at src/Fields.jl:155 [inlined]
apply! at src/operators/divergence.jl:82 [inlined]
...

Likely cause, and the mitigation taken

The rule bodies allocated: the hit counters were a Dict{Symbol,Threads.Atomic{Int}} looked up by key inside augmented_primal, and the augmented-primal helper took a closure. Rule bodies are compiled into Enzyme's generated code, so a dynamic lookup or GC allocation there is in exactly the wrong place.

Two changes landed together, so the cause is not isolated:

1. the counters became plain const atomics reached by static field access, and the closures went away;
2. the single-grid apply_bc!/fold_bc! rules were dropped entirely.

(2) alone would have fixed the crash, since it removes the offending rule. So we have a working tree but not a confirmed root cause.

Dropping (2) was independently justified: single-grid apply_bc! is 2·N·halo slab broadcasts that Enzyme already tapes correctly and cheaply, so a rule there was risk without measurable gain. The descriptor sweeps are where taping actually hurts, and those kept their rules.

Worth doing

• Reproduce on Linux x86_64 with an allocating rule body to confirm allocation is the trigger, and file an MWE upstream if so — "rule bodies must not allocate" is not documented anywhere I could find, and it fails as a segfault rather than an error.
• If the cause is confirmed and fixed, reinstating the single-grid rule is still optional; measure before bothering.

Note the platform asymmetry is the same one that makes #26 hard to work on: none of this reproduces on macOS/aarch64.

_{🤖 Filed by Claude, who would like the record to show it crashed the CI honestly and in public.}