Let Rust registrations override prelude definitions

[shsms]

A name first defined by a Lisp defun — notably the built-in prelude — registers its compiled-call dispatch in the compiler's vm_compilers.functions table and its CompiledDefun in bytecode.functions. compile_form consults that table before the symbol's global cell, but ctx.defun / defspecial / defmacro write only the global cell. So a direct call compiled on the VM path kept resolving to the prelude definition and the Rust override was silently shadowed — funcall/apply and the tree-walker already honored it, which made the asymmetry confusing.

Evict the stale compile-dispatch entry when a Rust function or macro is registered, so code compiled afterward resolves against the new global binding. The VM machine's own bytecode.functions is left intact, so the prelude's internal callers keep their pre-compiled binding.