CalculiX target: dlopen'd .so material (no ccx recompile per material)

[petlenz]

Adds CalculiXExternalTarget (make_target("calculix")) — codegen's first target whose output runs in a real external FE solver. It emits <Model>_ext.cpp compiling to lib<MODEL>.so, which CalculiX loads at runtime via dlopen: build ccx once with external-behaviour support, then each new material is just a new .so selected by name in the deck — no ccx recompile per material.

Supersedes the earlier two-PR split (#125 compiled-in umat_user_ + #126 external): the compiled-in approach needed a ccx relink per material and is dropped in favour of the .so path.

Mechanism (verified against ccx 2.22 source)

A deck *MATERIAL, NAME=@<LIB>_<FUNC> makes ccx (which uppercases the name) dlopen("lib<LIB>.so") + dlsym("<FUNC>") (external.c, -DCALCULIX_EXTERNAL_BEHAVIOURS_SUPPORT -ldl). The STANDARD interface passes native quantities under Abaqus-flavored names — STRAN1=emec (tensorial), STRESS=stre, DDSDDE=stiff(21) packed — handled by a tmech abq_std Voigt boundary + column-major stiff(21) packing (src/targets/calculix_boundary.h). The emitted plugin is a per-thread lifecycle: a thread_local stateless evaluator (ccx runs the element loop multi-threaded) that reads the material constants from MPROPS on every call (ccx interpolates them by temperature).

Verification

• CI (no external deps): calculix_check_driver calls the emitted NCG_UMAT through the exact calculixptr ABI and checks stre/stiff vs an independent isotropic oracle + a packing-order negative control + a read-constants-every-call regression + the icmd==3 stress-only path. Full suite 275 + 7 green.
• Real ccx (manual, examples/calculix/): build ccx once with external support, compile the codegen'd .so, run by @-name. Confirmed S11=0.027, S22=S33=0.013 (uniaxial) and S12=0.007 (simple shear — pins the tensorial convention).

Review

Went through a 3-lens review (architecture · C++/ABI · correctness); REVIEW.md has the findings. Fixes folded in: per-call MPROPS (was a stale thread_local cache — CRITICAL), symmetry guard (reject non-symmetric deformation-gradient inputs), shared boundary helper, shear-convention validation deck, single-table factory, and hardened build scripts.

Scope

Stateless (elastic). The STATEV round-trip for stateful J2 (numsim-materials-backed evaluate()) is the next step; the per-thread evaluator scaffold is already in place for it.

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