engine: MDL writer emits zero-arg builtin calls like PI() that the MDL parser rejects (Vensim has no PI builtin) #850

Problem

The MDL writer prints zero-argument builtin calls such as XMILE pi() as PI(). This output is doubly wrong:

1. simlin's own MDL parser rejects it on re-parse: symbol call requires at least one argument.
2. Vensim has no PI builtin at all, so the exported file is invalid Vensim MDL regardless.

In src/simlin-engine/src/mdl/writer.rs, mdl_bare_keyword (~line 181) only handles time, dt/time_step, starttime/initial_time, and endtime/stoptime/final_time; pi (and any other zero-arg builtin) falls through to the generic uppercase-function path and prints as NAME().

Impact

Part of the 29/240 re-parse failures found by a corpus sweep (parse -> project_to_mdl -> re-parse over all test/**/*.mdl under 200KB). Affected fixtures include:

• test/metasd/industrial-dynamics/IDch15/IDch15d.mdl
• test/metasd/thyroid/thyroid-2008-d.mdl

Component

src/simlin-engine/src/mdl/writer.rs (MdlPrintVisitor::walk, mdl_bare_keyword)

Possible approach

Emit a numeric literal for pi() (e.g. 3.14159265358979), matching what Vensim models actually contain. More generally, audit all zero-arg XMILE builtins reachable through the writer and give each an explicit MDL rendering (bare keyword, literal, or a documented reject) instead of the generic NAME() fallback. The corpus re-parse sweep from #847 would gate this too.

Related: #847 tracks the other (larger) re-parse failure class, wildcard subscripts printed as * instead of Dim!.

Context: identified during an MDL writer round-trip corpus sweep.