feat: support inheritance split and cast statements

[hjotha]

Summary

Adds first-class MDL support for type-based microflow decisions and cast actions:

split type $Input
case Sample.SpecializedInput
  cast $SpecificInput;
else
  return false;
end split;

Root Cause

The SDK layer could parse InheritanceSplit and CastAction objects from MPRs, but MDL had no AST or grammar nodes for them. The writer also did not serialize inheritance split/case/cast data back to valid microflow BSON, so describe/exec round-trips could not preserve these graphs.

Fix

• Adds InheritanceSplitStmt, InheritanceSplitCase, and CastObjectStmt to the microflow AST.
• Extends the grammar and visitor to parse split type branches and cast actions.
• Builds InheritanceSplit graphs with InheritanceCase sequence flows.
• Describes existing inheritance split graphs back to MDL by resolving case entity names.
• Serializes inheritance split objects, inheritance case values, and cast actions in the MPR writer.
• Updates validation, layout, reference walking, docs, skill guidance, and doctype examples.

Validation

• go test ./mdl/visitor -run 'TestMicroflowParsing_(Inheritance|Cast)' -count=1
• go test ./mdl/executor -run 'Test(FormatActivity_InheritanceSplit|FormatAction_CastAction|Builder_InheritanceSplit|TraverseFlow_InheritanceSplit|LastStmtIsReturn_InheritanceSplit|ValidateMicroflow_InheritanceSplit)' -count=1
• go test ./sdk/mpr -run 'Test(BuildSequenceFlowCase_InheritanceCase|SerializeMicroflowObject_InheritanceSplit|CastAction_RoundtripVariableName)' -count=1
• ./bin/mxcli check mdl-examples/doctype-tests/inheritance_split_statement.test.mdl
• make build
• make lint-go
• make test

Closes #348
Part of #332