Add a test fixture that pins the UDT operator execution path - #592

Each auto-lifted UDT operator carries both a JIT C definition and a Numba
cfunc, and SuiteSparse chooses between them per call depending on whether a C
compiler is available. A result measured on one path is not evidence about
the other, so a machine with a compiler and a machine without run different
code, and a single-path test reports on whichever the runner happened to get.

udt_op_path parametrizes over both and pins jit_c_control for the
duration of the test. Two details there are load-bearing:

• It sets the control rather than reading whatever is in effect. SuiteSparse
  demotes on to load after a failed compile, and a demoted control routes
  to the cfunc silently, so a jit cell would run the cfunc and pass.
• It re-reads the control before restoring it and fails the test when it was
  demoted mid-test. No assertion inside the test can see that happen.

_jit_can_compile memoizes whether a usable compiler exists. It deliberately
does not consult jit_c_control, which is the per-test state the fixture
owns; folding the two together is what let a demoted control go unnoticed.

This adds no production code and no test cells. A parametrized fixture
creates cells only for tests that request it, and nothing requests it yet, so
the suite count is unchanged. It lands on its own so that each commit which
goes on to consume it carries only its own change in operator semantics.

_jit_can_compile asks _enable_jit_for_udt(), the call the UDT auto-lift path makes, which does one real compile per process. Importing graphblas.ss no longer probes the compiler (#630 now sits below this PR), so reading jit_c_control == 'on' as the probe's answer skipped the jit cells whenever nothing had enabled the JIT first.

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}