CalculiX external-behaviour target: dlopen'd .so plugin (no ccx recompile)

[petlenz]

Stacked on #125. Adds CalculiXExternalTarget (make_target("calculix_external")) — emits <Model>_ext.cpp compiling to lib<MODEL>.so that CalculiX loads at runtime via dlopen, so adding a new material needs no ccx recompile (build ccx once with external support; select materials by name in the deck).

Mechanism (verified against ccx 2.22 source)

A deck *MATERIAL, NAME=@<LIB>_<FUNC> makes ccx (which uppercases the name) dlopen("lib<LIB>.so") and dlsym("<FUNC>") (external.c, gated on -DCALCULIX_EXTERNAL_BEHAVIOURS_SUPPORT + -ldl). The STANDARD interface passes native quantities under Abaqus-flavored names — STRAN1 = emec (tensorial), STRESS = stre, DDSDDE = stiff(21) packed — so the same abq_std boundary as the compiled-in target (#125) reuses verbatim. The emitted plugin follows a per-thread lifecycle: a thread_local material instance built once per thread (ccx runs the element loop multithreaded), then per-call unpack → evaluate → pack.

Verification

• In-repo gate (no deps): calculix_check_driver gains 3 tests that call the emitted NCG_UMAT through the exact calculixptr ABI and check stre/stiff against the isotropic oracle, plus an equivalence test proving the external entry matches the compiled-in umat_user_ bit-for-bit. Full suite 272/272.
• Real ccx run: examples/calculix/build_and_run_external.sh builds ccx once with external support, compiles the codegen'd .so, and runs uniaxial_c3d8_external.inp by @-name. Confirmed S11=0.027, S22=S33=0.013 — same result as the compiled-in path, with no ccx recompile.

Scope

Stateless (elastic) — same guards as #125. The STATEV round-trip for stateful J2 (numsim-materials-backed evaluate()) is the next step; the thread_local ②/per-call ③ scaffold is already in place for it.

🤖 Generated with Claude Code

[petlenz]

Consolidated into a single PR against main (external .so target only).