ci(mir): run the Michaelis-Menten sensitivity tests, and correct the Windows note

.github/workflows/mir.yml

@@ -22,12 +22,21 @@ name: mir-jit
 # from-source SuiteSparse build. The KLU-gated codegen tests (sparse Jacobian)
 # skip cleanly on a KLU-off core.
 #
-# On Windows the default cc+dlopen codegen path is not the story — a bare
-# Windows box is the case with no C compiler, which is exactly why MIR exists
-# (#3) — so the cc-baseline tests (marked needs_cc: cc/clang/gcc) skip there and
-# test_mir_equivalence's compiler-free MIR-vs-ExprTk check is the end-to-end
-# Windows verification. On Linux/macOS cc is present, so cc-vs-MIR bit-identity
-# is asserted too.
+# Why Windows is in the matrix: a bare Windows box is the case with no C
+# compiler, which is exactly why MIR exists (#3).
+#
+# The GitHub `windows-latest` *runner* is not that box, though. It ships MinGW
+# gcc on PATH, so `_CC = which("cc") or which("clang") or which("gcc")` resolves
+# and the `needs_cc` cc-baseline tests do NOT skip there — measured, 0 skipped on
+# every leg. All four legs therefore assert the strong property, cc-vs-MIR
+# bit-identity on the same generated source, not just the compiler-free
+# MIR-vs-ExprTk comparison. (An earlier version of this comment claimed the
+# cc-baseline tests skip on Windows and that MIR-vs-ExprTk was the whole of the
+# Windows verification. Both were wrong, and they understated the coverage.)
+#
+# The consequence worth remembering: this job does not exercise the
+# no-compiler-at-all path that motivates MIR. If that path needs guarding, it
+# needs a leg with the compilers taken off PATH, not this one.
 #
 # The three Functional analytic-sensitivity files (#66's ∂f/∂p, #67's fused J·yS,
 # #68's clock-conditional laws) are in the list below, but their end-to-end half
@@ -41,6 +50,16 @@ name: mir-jit
 # the quarantine removes itself: fixing #85 turns them XPASS and this job red
 # until the markers come out. They are the coverage that found #85, and #67's
 # first attempt at this list is what surfaced it.
+#
+# The Michaelis-Menten file (#55's MM stage) needs no such marker, and the reason
+# is worth stating rather than rediscovering: #85 fires on
+# bngsim_codegen_output_sens, which generate_output_sens_from_model only emits
+# when the model has at least one *function*. The MM fixtures declare none, so
+# the symbol is absent even with emit_output_sens forced on, and their
+# Simulator-with-sensitivity_params cases compile here. c2mir already handles the
+# arithmetic: mm_tqssa.net goes through test_mir_equivalence and
+# test_codegen_jacobian on this job today, and the MM sensitivity block is the
+# same closed form as the Jacobian block, from the same builder.
 
 on:
   workflow_dispatch:
@@ -133,6 +152,7 @@ jobs:
             python/tests/test_codegen_functional_dfdp.py \
             python/tests/test_codegen_functional_sens_rhs.py \
             python/tests/test_codegen_switch_condition_sens.py \
+            python/tests/test_codegen_mm_sens.py \
             python/tests/test_codegen_determinism.py \
             -rA --tb=short ${{ github.event.inputs.pytest_args }} \
             2>&1 | tee mir-${{ matrix.os }}.log