Root cause: sf-letrec returns a thunk (for TCO) but the CEK dispatch
wrapped it as a value without evaluating. The thunk leaked as the
return value of letrec expressions, breaking sx-parse and any function
using letrec.
Fix: step-sf-letrec unwraps the thunk into a CEK state, so the last
letrec body expression is properly evaluated by the CEK machine.
Also:
- compile-letrec: two-phase (nil-init then assign) for mutual recursion
- Skip JIT for inner functions (closure.bindings != globals) in both
vm_call and JIT hook
- vm-reset-fn for sx-parse removed (no longer needed)
- Parser regression test: letrec with mutable pos + recursive sublists
Test results: JS 943/17, OCaml 955/0, Python 747/0
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
giles
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