[Montana]

Closes #1335.

#1335 lists three paths that #1302 (#1002) left without a test or a decision. No wrong answer was known on any of them, and this PR finds none. It pins each one and documents the third.

1 and 2: the two steady-state quotients with no jump mend

#1302 mends a difference-quotient row whose step crosses a jump for runs (dense and colored) and for the dense steady-state march. Two quotients were left as they were:

• KINSOL's own quotient: method="newton" with jacobian="fd". A declined law sends newton to the march instead, so that path is reached only with "fd".
• The steady-state march's colored sparse quotient: KLU.

New tests in test_difference_quotient_jump.py run each path, plus KINSOL on KLU, from:

• a start on a threshold, and 1e-9 above and below it, on X' = s − kx·k(X)·X with k = 0.04 above 5 and 0.02 at or below. With s = 0.08 the one steady state is X = 4, below the threshold; with s = 0.4 it is X = 10, above;
• beside a species held at 0, using the two The differenced state Jacobian steps every species by 1.5e-8 of the largest concentration: dY_ss/dp with jacobian="fd" is 7% off beside an unrelated species at 1e7 and 44% at 1e8, with no error #1002 models that broke runs: the gate (Y = 0.5) and the inhibitor (Y = 20).

Each is checked against its closed form (< 1e-4 relative, the file's OK), and the held species against exactly 0.

Each test also asserts the path it took: method_used, linear_solver (dense/klu) and solver_jacobian_source == "finite-difference". Prototyping showed why that matters: with a declined law, method="newton" silently converges by the march instead, and would have "tested" KINSOL without touching it.

	cases	on main
threshold starts × 2 sources × 3 paths	18	all pass
held species × 2 models × 3 paths	6	all pass

The KLU paths skip on a KLU-off build, with the reason conftest.py already declares.

These pin the current behaviour rather than fix a defect. They are the "nothing pins that" #1335 asks for, and they agree with #1302's 72 and 24 variants that found no wrong answer.

3: SUNDIALS other than the vendored 7.2

include/bngsim/dq_jacobian.hpp reproduces cvLsDenseDQJac of the vendored SUNDIALS (v7.2.1) bit for bit. With -DBNGSIM_USE_SYSTEM_SUNDIALS=ON and another release, it is still a valid quotient, but not necessarily that release's own. #1335 asks for a CI leg or a documented refusal. This does the documented half:

• CMakeLists.txt: in the system-SUNDIALS branch, compare SUNDIALS_VERSION's major.minor with VENDOR.json's authoritative_release_tag, and warn on a mismatch:

  CMake Warning: System SUNDIALS 7.3.0 is not the vendored v7.2.1.  bngsim's dense
  difference quotient (jacobian="fd", or a law the closed form declines) reproduces
  CVODE 7.2's.  With this release it is still a valid quotient, but can differ from
  this CVODE's own in the last digits, and no CI leg tests the combination (issue #1335).
  

• docs/installation.md: the same note beside BNGSIM_USE_SYSTEM_SUNDIALS.

Verified by running the block in cmake -P with SUNDIALS_VERSION set to each of these:

version	warns
7.2.1	no
7.2.0	no
7.3.0	yes
6.7.0	yes
unset	yes

The default (vendored) configure does not enter the branch. Every CI leg uses the vendored path, so the full configure is exercised there.

A warning, not a refusal: a system SUNDIALS of another 7.x is supported (find_package(SUNDIALS 7.0 …)), and refusing it would drop HPC builds that link a site SUNDIALS.

Measured

test_difference_quotient_jump.py with the new cases, plus test_skip_audit.py, test_ci_run_list_coverage.py and test_docs_api_examples.py: 120 passed.