fix(mdl): refuse throw, float/currency/date and the parenthesised association; warn on unstored index names and enum-value docs

[ako]

Closes #706. Plan item 0.3 (the §2 silent drops) of PROPOSAL_mdl_beta_syntax_freeze.md, tracked in #714.

What was wrong

Before this change, each of these got ✓ Syntax OK and Check passed! from mxcli check, and was then lost or stored wrongly:

Form	What happened	Now
throw 'x';	no listener, so the statement vanished from the microflow	error. The message points to raise error; inside an on error handler, or a Java action on the main flow
Amount: float / currency	stored as String(unlimited), or Void as a microflow type	error: use Decimal
Born: date	stored as DateTime	error: use DateTime (Mendix has no date-only type)
create association X (from A to B, type: referenceset, storage: table)	every option dropped, so it was stored as a Reference with column storage	error. The message rewrites the statement in the working form
/** … */ on an enumeration value	read into the AST and never written	warning MDL-ENUMDOC01
index name (index Idx on (…), add index Idx (…), create index Idx on E (…))	discarded	warning MDL-IDX01

Design choices the ADRs did not settle

• Errors when the model would be wrong, warnings when only text is lost. throw, the types and the association form each write a different model, so they are refused. An enumeration-value doc comment or an index name leaves a correct model and only loses text. Our own examples used enum-value doc comments in 56 places, and create index requires a name. Both therefore warn: through ValidateProgram, exec prints the warnings as well. Making them errors under an mdl 1; header can come later through the deprecation registry.
• Refused from the parse tree; the grammar is not deleted. ADR-0010 R11 says grammar that can never succeed should be removed. But float, currency, date and throw are also accepted as keywords-as-identifiers. Deleting | FLOAT_TYPE from dataType would let Amount: float parse again as an enumeration reference named float, which is the same silent failure. The alternatives stay as rejection anchors with exact line:col errors. Proposal item 2.3 can delete them once a hint mechanism for removed syntax exists.
• The type check is global. It runs on EnterDataType/EnterNonListDataType, which covers attributes, microflow parameters and returns, declare, constants and the service rules. It looks only at the direct token, so M.Currency and enumeration(M.Float) still parse (there is a control test for this).
• Enum-value doc comment hint: use a -- comment, or the enumeration's own /** */.

Test plan (what I ran)

• Tests written first and seen failing against unfixed code: mdl/visitor/silent_drops_706_test.go and mdl/executor/validate_unstored_text_test.go. Every rejection test has a control next to it (canonical association keeps ReferenceSet/Table, decimal/datetime/M.Currency still parse, the enumeration's own doc comment is stored, anonymous indexes and drop-by-columns do not warn).
• Revert check for each fix. Disabling ExitThrowStatement, EnterDataType/EnterNonListDataType, the LPAREN branch of the association visitor, the validateIndexNames / validateEnumValueDocs wiring, or the index-name / value-doc capture makes the matching test fail. Restoring the fix makes it pass.
• End to end: before the fix, mxcli check on a script with all six forms printed "Syntax OK (5 statements) / Check passed!". After the fix it gives four errors with the replacement spelling, and the two text-only forms give warnings.
• Sweep: ran the old and new binaries' check over every file in git ls-files '*.mdl' and compared exit codes. Five example files depended on the dropped forms. They are fixed in the second commit; the widget demo's association now really is the ReferenceSet it declared.
• make build, make lint and make check-findings pass. In make test, every package passed except mdl/backend/modelsdk, which hit Go's 10-minute default timeout (load average above 20, with several agents compiling on the same machine). Rerun alone with -timeout 40m it passes (ok … 660.9s). This change does not touch that package.
• Finding appended to .claude/skills/fix-issue/findings/mdl-visitor.jsonl.
• Not run in Studio Pro: nothing new is written. Every change is a refusal or a warning, or an example that now uses a form that was already supported.

Follow-ups (not in this PR)

• ast.TypeDate and its writer, describer and diff branches are now unreachable from the parser and can be deleted.
• A legacy constant of DataTypes$FloatType describes as type Float, which is now refused on re-exec. It used to become a String silently. If such models exist in the wild, describe should map it to Decimal.
• The declared name on AutoOwner-style attributes (proposal §2 item 11) is covered by MDL022 and was not changed here.

🤖 Generated with Claude Code