Add an .optional arm to regToValue in comptime_vm.zig: read the
has_value flag at offset sizeof(child), bridge the payload recursively
into a { payload, i1=true } aggregate when set, yield .null_val (zero
{T,i1}) when clear or the bare null sentinel. Matching serialize arm in
serializeAggregateValue (emit_llvm.zig). Pointer/?Closure/?Protocol-child
optionals and array-payload aggregates bail loudly, not silently.
Regression: examples/comptime/0643-comptime-run-optional-aggregate.sx
(present ?T, present ?i64, null ?i64). Verified by 3 adversarial reviews.
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