Prerequisite for wiring the VM at the lowering-time comptime site
(runComptimeTypeFunc), where IR can be malformed (an unresolved name lowers to a
dangling / Ref.none operand — the 0737 crash). Close the remaining panic vectors
so the VM bails (-> legacy fallback) instead of aborting:
- Vm.refTy(ref_types, r): a bounds-checked accessor replacing every raw
ref_types[ref.index()] in exec — the type-side companion to Frame.get's
bad_ref value-side guard.
- aggType is now a bailing method (Error!TypeId) routed through refTy.
- the block-dispatch loop bounds-checks the branch target before indexing
func.blocks.items (a malformed br target). global_get was already guarded.
No behavior change: gate OFF and -Dcomptime-flat both 697/0. Unit test added
(a cmp_lt with a Ref.none operand bails, not panics).
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