Review: fix: emit expression case values for IF flows

Verdict: Approve with minor notes

What the PR does

Fixes issue #491: mxcli was serializing IF-branch sequence flows as Microflows$EnumerationCase (string "true"/"false") when Studio Pro 10+ expects Microflows$ExpressionCase (field Expression). The fix introduces caseValueForFlow to correctly produce *ExpressionCase for boolean branches and EnumerationCase for real enum values, with a version-gate in buildSequenceFlowCase that downgrades ExpressionCase → EnumerationCase when writing Mendix 9 projects.

No blockers

Moderate concern

No DESCRIBE roundtrip test (Recurring Finding #4)

The PR adds unit tests for caseValueForFlow and buildSequenceFlowCase, but no test that builds a microflow with an IF split, serializes it, re-parses it, and confirms DESCRIBE produces correct MDL. This is exactly the gap the roundtrip finding covers. A failing Studio Pro project won't fail make test, so the regression can resurface silently if the new case type is mishandled anywhere in the show/format path.

Suggested: add a test in mdl/executor/ that creates a microflow with an IF branch, calls the BSON serializer, re-parses, then calls showMicroflowActivities and asserts the output contains if ... then rather than garbage.

Minor issues

flowCaseString handles only pointer *ExpressionCase, not value form

The updated flowCaseString test helper only matches *microflows.ExpressionCase. If any test creates a value-form ExpressionCase{} directly and passes it through flowCaseString, it silently returns "". In practice all production paths (caseValueForFlow, parser) produce pointers, so this is unlikely to bite, but it's inconsistent with how EnumerationCase is handled (both forms).

Positive notes

• Clean PR — no bundled unrelated changes
• The existing findBranchFlows already handled *ExpressionCase pre-PR; BooleanCase handling retained for legacy MPR files
• Version gate (majorVersion <= 9) correctly aligns with the NewCaseValue vs CaseValues[] serialization split already in the writer
• Two new writer tests explicitly cover Mx9 downgrade and Mx10+ path — this is exactly the right level of coverage for version-specific serialization