Symptom: enumeration decision splits could not be represented as first-class MDL, so describe/exec round-trips either collapsed them into boolean-looking splits or lost the enumeration case structure.

Root cause: the microflow AST, parser, visitor, builder, describer, validator, and reference collectors only modeled boolean IF-style exclusive splits. Sequence flows with EnumerationCase values had no statement form.

Fix: add a split enum statement with case/else bodies, emit and parse enumeration case values, build ExpressionSplitCondition graphs with EnumerationCase flows, describe existing enum split graphs back to MDL, and teach validation/layout/terminality/reference walks to recurse through enum branches.

Tests: added parser, builder, describer, terminality, and validation regression tests plus a doctype fixture checked with mxcli check. Also ran make build, make lint-go, and make test.

Symptom: an enum split branch containing a nested empty-then decision could
round-trip to an MPR whose outgoing decision flow lacked the boolean case value,
causing Mendix mx check CE0079/CE0773 even when the described MDL was stable.

Root cause: addEnumSplit consumed flowBuilder.nextConnectionPoint from the
nested decision but dropped flowBuilder.nextFlowCase before wiring the branch
continuation to the shared enum merge.

Fix: carry the pending case value through enum branch continuation wiring and
reapply it when connecting to the next statement or enum merge.

Tests: add a builder regression for a nested empty-then enum branch that must
keep CaseValue=true on the continuation flow.