Deduplicate target emission helpers (#139)

[petlenz]

Addresses #139 (largest items; two smaller ones remain — see below).

STACKED PR — base is fix-137-can-emit (PR #158), NOT main. Merge #158 first, then retarget this to main (REST PATCH, not gh pr edit — the latter silently no-ops on this org) or merge as-is after #158 lands.

• Intra-file (numsim_material.cpp): the rate and residual paths' duplicated blocks extracted into file-local helpers — collect_scalar_params/collect_tensor_inputs (CollectedSymbols + structured bindings), reject_non_finite_default, emit_parameters_schema(...) parameterized on the solver_source/integrator_source difference, and MemberUniquenessGuard::claim.
• Cross-target: uses_linear_algebra(body, la) (linear_algebra_emitter.h) and uses_spectral_runtime(body) (spectral_decompose_emit.h) replace the verbatim include-gating logic duplicated across standalone_cxx.cpp and moose_material.cpp.

Emitted text proven byte-identical: full suite green with zero test edits (the exact-substring assertions + compile-check drivers are the oracle), plus a diff -r of the standalone registry output regenerated before/after — clean.

Remaining #139 scope (follow-up): the EmittedOutput/register_* local-structure pair in numsim_material.cpp.

gcc-14 Debug (-Werror): ctest 331/331.

[petlenz]

No CI checks appear here by design: build.yml triggers only on PRs targeting main, and this PR's base is fix-137-can-emit (stacked). Validated locally (gcc-14 Debug -Werror, ctest 331/331). Once #158 merges and this is retargeted to main (REST PATCH), the full matrix runs.