The compiler was treating letrec as let — binding values sequentially.
This meant mutually recursive functions (like sx-parse's read-list
calling read-expr and vice versa) couldn't reference each other.
compile-letrec uses two phases:
1. Define all local slots initialized to nil
2. Compile and assign values — all names already in scope
This fixes sx-parse producing wrong ASTs (nested instead of sibling
lists) when JIT-compiled, which caused the stepper's step count to
be 2 instead of 16.
Also: skip JIT for lambdas with closure bindings (inner functions
like read-list-loop) — the closure merging into vm_env_ref produces
incorrect variable resolution.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
giles
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