Parser: function calls compose as call args

[danieljohnmorris]

Deferred follow-up from PR #159. Doc-cited hd tl xs doesn't chain; prnt str nc parses as prnt(str, nc) (2 args). Single-pass with forward references: builtin arities pre-registered, user fn arities registered as their headers parse. New parse_call_arg(in_fn_ref_pos) eagerly consumes nested calls when the inner is a known function with N args. Outer arity not clamped so over-arity verifier errors still surface. HOF args (fld max xs 0) preserved via fn-ref slot flag. 14 cross-engine tests + example.

[codecov]

Codecov Report

❌ Patch coverage is 94.47853% with 9 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/parser/mod.rs	94.37%	9 Missing ⚠️

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