materializeDefaultContext now falls back to a zeroed Context (found by name) when
the __sx_default_context global is absent — i.e. at lowering time, where the global
isn't emitted yet. A type-fn that never touches the allocator runs past context
setup; one that allocates reads a null alloc_fn (zeroed) and call_indirect on the
null func-ref bails to legacy (a real lowering-time context with the CAllocator
thunk func-refs is a follow-up).
Measurement (SX_COMPTIME_FLAT_TRACE): the bail moved deeper — make_enum now bails
at const_type (the Type-literal op, unported); register_type type-fns bail at the
welded write call. No table mutation before either bail (write fns bail before
minting), so parity holds: both gates 697/0, no crashes.
Next: model the const_type op + the Type-return bridge + the VM-native write side,
which together let a type-fn run end-to-end on the VM.
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