engine: variable named 'inf' (or other zero-arity builtin name) collides with the Inf builtin and silently reads +infinity #681

Summary

A user variable named inf is resolved as the zero-arity Inf builtin in expression position. In src/simlin-engine/src/ast/expr1.rs (~line 203) the lowering maps the bare identifier inf to BuiltinFn::Inf ("inf" => check_arity!(Inf, 0)) before checking whether a model variable shadows that name. As a result, any equation referencing a variable named inf reads +infinity instead of the variable's value, silently poisoning downstream results with Inf/NaN.

This bites LTM-synthesized helpers too: an LTM PREVIOUS-helper aux built over a model variable named inf reads +infinity, so link scores through it come out NaN. It was found while building LTM test models -- a flow named inf produced NaN link scores -- but the root cause is not LTM-specific; it affects any model with a variable named inf (and likely the other zero-arity builtin names such as pi, nan, etc.).

Why it matters

This is silent numeric corruption. The model compiles without error, and the user sees Inf/NaN propagating through results with no diagnostic pointing at the shadowed name. The name is unusual, which lowers the practical frequency, but the failure mode (no error, wrong numbers) is severe when it does occur.

Component(s) affected

• src/simlin-engine/src/ast/expr1.rs (~line 203, builtin lowering for zero-arity builtins)
• Anything that references such a variable, including LTM-synthesized PREVIOUS-helper auxes.

Possible approaches

• Only treat inf / pi / other zero-arity builtins as builtins when no model variable shadows the name (resolve the variable first, fall back to the builtin).
• Or reject such a variable name at parse time with a clear error so the collision is loud rather than silent.

Severity

Minor-to-major -- silent numeric corruption (Inf/NaN), but triggered only by an unusual variable name.

Context

Found while building LTM test models during a 2026-06-02 design review (a flow named inf produced NaN link scores). Not LTM-specific.