materials: decompose the elastic stiffness into a material the graph can order

[petlenz]

linear_elasticity computes stress and tangent in one material, so its tangent is invisible to the property graph: reading a property inside the material that owns it creates no edge, and elastic::stress happens to sort before elastic::tangent. Anything that wanted the stiffness to follow a changing input would silently lag by one call.

Split it in three, so every dependency is an edge the engine can see:

material	publishes	inputs
constant_scalar	value (plain property)	—
isotropic_tangent	tangent = C(K, G)	K, G as Global
linear_stress	stress = C : ε	tangent, strain as Global

The moduli being graph inputs is the point. A constant is one node today and a host-bound or temperature-dependent source tomorrow, with no change to the two materials downstream.

isotropic_tangent recomputes on every update rather than memoising. It has one job; the memo measured 28.1 ns against 27.0 ns for the monolithic material; and a validity flag is state that can disagree with its inputs.

material_point_evaluator::config::tangent_source

With the stiffness in its own material, stress and tangent no longer share an owner. The field is std::optional<std::string> so unset and "" stay distinct, and it is appended, not inserted — this header is embedded in third-party UMATs, and a new field in the middle would silently re-bind the trailing arguments of an existing aggregate initialiser.

Tests

10, including that the decomposed pair reproduces linear_elasticity through the evaluator exactly — the decomposition has to be a refactor of the physics, not a new model.

Split out of #26.

[petlenz]

Critical review — probed rather than read. Nothing found. Recording what I tried to break so the absence of findings means something.

Silent-default holes: none. isotropic_tangent marks K_source and G_source is_required, and linear_stress marks tangent_source and strain_source is_required. There is no path where a missing wire degrades into a plausible default — the failure is at construction, by name.

The ABI claim about config::tangent_source holds. I checked the struct rather than trusting the comment: it really is the last member, so an existing aggregate initialiser cannot have its trailing arguments re-bound.

std::vector<statev_exclusion> extra_exclusions{};
std::optional<std::string> tangent_source{};   // last

A missing tangent owner fails loudly. linear_stress publishes only "stress", so using it as a stress source with tangent_source unset makes the evaluator resolve elastic::tangent, which does not exist — fatal_error naming the property, not a zero tangent. That is the right side of the line for a setup fault.

Dropping the memo is safe under repeated evaluation. The concern with recompute-every-update was a material inside a solver's inner loop recomputing needlessly; K and G do not change across those iterations, so the result is identical and the cost is the 28.1 vs 27.0 ns already measured.

One thing I deliberately did not treat as a finding: linear_stress computes sigma = C : eps from TOTAL strain, which is only the constitutive law for linear elasticity. That is what the name says, and the tests pin it against linear_elasticity exactly, so it is a correct narrow material rather than an incomplete general one.

Findings on the rest of the stack: two on #30 (one high), two notes on #29, one diagnostic regression on #27.

[petlenz]

Second review. No correctness findings — this PR came back clean last time too. One documentation accuracy point inline.

[petlenz]

This number compares against code that no longer exists.

"~309 ns against ~28 ns memoised" — the memoised variant was removed in this PR, so a reader cannot check the comparison or reproduce it. It is a claim about a counterfactual.

Re-measured what is actually here, 300k iterations each:

isotropic_tangent alone, per update :   286.8 ns
decomposed pair, per update         :   315.0 ns
linear_elasticity, per update       :    26.1 ns

So 309 was in the right area for the rebuild itself (286.8 here), but the useful comparison is the last two lines: 315 ns against 26 ns, 12x on the whole graph, against a material that exists and that the same sentence already points at. That is the number someone deciding between the two would want.

The sentence's conclusion does not change — where the moduli are fixed, linear_elasticity is the cheaper choice — only the evidence offered for it. Worth swapping since a comparison against deleted code cannot be re-checked when either side drifts.

[petlenz]

Fixed. The header now quotes the comparison against a material that exists:

isotropic_tangent alone :  286.8 ns
decomposed pair         :  315.0 ns
linear_elasticity       :   26.1 ns

~315 ns against ~26 ns — 12x on the whole graph. Conclusion unchanged; the evidence can now be re-run when either side drifts, which a comparison against deleted code could not.