Recipe builders silently accept duplicate `add_*` calls

[petlenz]

Summary

ConstitutiveModel's add_scalar_input, add_tensor_input, add_parameter, and both add_output overloads do no name-collision checking. Duplicate names silently push duplicate entries into m_symbols / m_outputs, producing invalid C++ later:

• add_parameter("E", 200e3) twice → two double const E formal parameters → C++ redefinition error.
• add_output("sigma", expr1) then add_output("sigma", expr2) → two T_n & sigma_out parameters + two assignment statements → C++ error, plus the MOOSE backend declares the property twice → MOOSE runtime error.

The user gets a confusing downstream error instead of a clear recipe-time std::runtime_error("duplicate parameter 'E'").

Where

• include/numsim_codegen/recipe.h:94-104 (add_scalar_input)
• include/numsim_codegen/recipe.h:106-117 (add_tensor_input)
• include/numsim_codegen/recipe.h:121-133 (add_parameter)
• include/numsim_codegen/recipe.h:137-150 (both add_output overloads)

Fix

In every add_* method:

1. Check m_symbols for an existing entry with the same name (inputs/params share a namespace because they all become function parameters).
2. For add_output, also check m_outputs.
3. Throw std::runtime_error("duplicate <kind> '<name>'") on collision.

Pair with tests in tests/RecipeTest.cpp exercising each duplicate path. Worth also adding an identifier validator (reject C++ keywords, leading digits, names ending in _out) in the same pass — see the medium follow-up issue.

Refs

• REVIEW.md → CORR-A4 (High), CORR-24, CORR-25
• Severity: High