Target: add can_emit() capability query — per-target emit preconditions are discoverable only via exceptions

[petlenz]

From the 2026-08-02 full-repo review (finding F-10).

Evidence: targets/target.h:39 offers only emit(). Accepted recipe domains differ radically per backend: NumSimMaterialTarget rejects scalar inputs / multi-state / tensor state (numsim_material.cpp:87-143, 181-236); MooseMaterialTarget rejects stateful inputs, evolution-without-local-Newton, >1 tangent (moose_material.cpp:456-509); StandaloneCxx accepts nearly everything. A generic driver (e.g. recipe_registry_gen) can only find out by catching std::runtime_error — the direct cause of the registry-generator abort issue.

Proposal (additive, non-breaking): [[nodiscard]] virtual auto can_emit(ConstitutiveModel const&) const -> std::expected<void, std::string>; default implementation may conservatively return success ("try emit"). Concrete targets refactor their existing guard blocks to feed both can_emit and emit from one place, keeping the excellent reject-loudly messages.

The "reject loudly, never silently drop" policy itself is right and should stay — this only makes rejection queryable. Related: #93 (factory formalization), registry-abort issue.

[petlenz]

Follow-up from the PR-batch critical review (2026-08-02): examples/recipe_registry_gen.cpp now wraps target->emit() in a blanket catch (std::exception) to implement skip-and-continue (#135 / PR #149). That is deliberately too broad — it also converts unexpected defects (std::out_of_range from a genuine pass bug) into a friendly "SKIPPED" line. When can_emit() lands, switch the generator to query it and let emit() exceptions propagate again — that restores the tool's value as a crash-loud smoke check while keeping the graceful skip for genuinely unsupported recipes.