Tracking: follow-up refactors for the reacting immersed-boundary surface chemistry (#1821) #1893

Tracking issue for the #1821 follow-ups, so the ordering is written down somewhere.

#1821 landed heterogeneous immersed-boundary surface chemistry as a single 2733-line change. It is reviewed and working, and splitting it retroactively would be churn — but the pieces inside it have different owners, different test strategies, and in one case a different repository.

Where the lines are

Ordering

1. Move the surface-chemistry Fortran codegen upstream into pyrometheus #1891 — codegen to pyrometheus. First: it deletes the most MFC code, and it is the only piece with an external review cycle. Everything else is unaffected by it.
2. Extract the surface-chemistry solver out of m_ibm.fpp into its own module #1892 — extract the solver module. Pure move plus unit tests; goldens must not shift.
3. Nothing else. The remaining ~1400 lines are an example, its mechanisms, generated goldens, and ordinary parameter plumbing. Those are the right size and in the right place — this is not an argument that Add heterogeneous reacting surface boundary conditions #1821 should have been four PRs.

Why bother

The two extractions are what make the feature independently testable. Codecov reports 56.97% patch coverage on #1821 with 55 uncovered lines in m_ibm.fpp; the solver and the codegen are precisely the parts currently reachable only through a full 2-D reacting simulation compared against a golden. Both have already produced bugs that unit tests would have caught cheaply (a sticking-coefficient rate emitted as plain Arrhenius; a sum-constraint closure landing on a trace radical).

No action needed on #1821 itself.