Preserve enum split microflow roundtrips

[hjotha]

Part of #332. Staged in draft PR #346.

Summary

Some legacy microflows use enum-based ExclusiveSplit conditions. describe microflow can flatten or misclassify those graphs because MDL currently has no explicit enum split statement that preserves enum case values, grouped cases, and terminal/shared-tail behavior.

Expected behavior

describe -> exec -> describe should preserve an enum split as structured MDL, including:

• the split source variable or attribute path;
• each enum case label, including grouped cases;
• an (empty) case when present;
• terminal branches and shared continuation tails;
• MPR validity after roundtrip.

Current staging status

The behavior is implemented in draft PR #346, but that PR is intentionally not merge-ready. This needs extraction into a focused PR with parser/AST/visitor/builder/describer/validator coverage and synthetic tests only.

Validation target

Focused PR should pass make build, make lint-go, make test, and at least one mxcli check doctype example for the new syntax.

[github-actions]

Issue Triage

Summary:
The issue requests a feature to preserve enum‑based ExclusiveSplit microflows when using describe → exec → describe, ensuring the split source, enum cases (including grouped and empty), terminal/shared branches, and MPR validity are maintained after a round‑trip.

Checklist (feature request):

• ✅ Use case – clearly explained (legacy microflows with enum splits lose information during MDL round‑trips).
• ✅ Proposed syntax or behavior – detailed expected behavior (preserve split source, each enum case label, (empty) case, terminal/shared tails, MPR validity).
• ⚪ Mendix version relevance – not indicated as version‑specific; if the feature applies to all Mendix versions, this field is not required.

Next steps:
If the feature is intended for a specific Mendix version range, please add that information. Otherwise, the issue looks complete—thanks for the clear description!

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