Direction correction (user): the VM does NOT get a transitional compiler_call →
hook-registry shim. BuildOptions is re-expressed as abi(.zig) extern compiler
functions (the compiler-API the VM already dispatches via callCompilerFn), and the
#compiler attribute + compiler_call op + Value-based hook Registry are deleted.
Also record that 4D (host FFI) is DONE via the arena/absolute-pointer allocator
(the earlier pin/tag hazard is moot — the arena never moves an allocation), and
that 0602/0603 stay on legacy fallback until the BuildOptions migration lands.
No code change (reverted the speculative compiler_call bridge).
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