feat(pages): write DynamicClasses and DynamicCellClass as expressions (#750)

[ako]

Relates to mendixlabs#750. This is slice 2 of docs/11-proposals/PROPOSAL_first_class_expressions.md, the named-property step. Q4 was decided on 2026-09-25: schema-driven in the end, shipped named properties first.

What changes

A widget's DynamicClasses and a datagrid column's DynamicCellClass hold one Mendix expression, and MDL now writes it as-is:

container c1 (dynamicclasses: if $currentObject/Featured then 'is-featured' else '') { }
container c2 (dynamicclasses: 'is-featured') { }       -- the string: the class is-featured
column Name (attribute: Name, DynamicCellClass: if $currentObject/Price > 100 then 'highlight' else '')
alter page M.P { set DynamicClasses = if $currentObject/Featured then 'a' else 'b' on c1 };

A quoted value is a Mendix string, the same rule the OData client's credentials follow since #676. The doubled-quote spelling 'if … then ''a'' else ''''' is gone.

How

• Grammar: widgetPropertyV3 and alterPageAssignment accept expression last, so every existing value form keeps its parse.
• Visitor:
  • The two properties store the source text of whichever value alternative matched. A $x/Y + … can be claimed by the datasource or action alternative, so the visitor doesn't rely on which alternative matched.
  • A bracketed list still goes to MDL-WIDGET27/32.
  • An expression on any other widget property, or in any other alter … set, is an error, not a silent empty value.
• describe: prints the stored expression as-is.
• MDL-WIDGET33: refuses the old spelling on create and alter. That is a quoted string whose content looks like expression text: a $, a quote character, or a leading if. It would now store that text as a class name, so the error's suggestion is the unquoted expression. Plain class names like 'btn btn-lg' pass.
• Messages: MDL-WIDGET32 and the ALTER setter's bracketed-list error now point at the unbracketed expression.

Evidence

Tests were written against the pre-change behavior. Controls: class-name strings, class lists, the unquoted expression and visible: [...] pass throughout.

Measured on a copy of ako/TestApp (Mendix 11.14.0):

• Appearance.DynamicClasses stores if $currentObject/Featured then 'is-featured' else '' and 'plain-class', and columnClass stores its expression.
• describe prints the same MDL, and describe output is byte-identical after a describe → exec.
• alter … set with the expression stores it. With the old spelling it is refused, and nothing is written.

Migration

28 uses were converted: four skills, the docs site, the MDL quick reference, the syntax help, and seven example / bug-test files.

• A script lexed each MDL string and rewrote only values whose content is expression text. I reviewed the dry run, and every converted .mdl still passes check.
• Skill prose that told people to double the quotes is updated. One alter-page example had become self-contradictory after conversion and was rewritten.
• Proposals and the changelog keep the old spelling as history.

New regression file: mdl-examples/bug-tests/750-dynamicclasses-legacy-quoted-expression.fail.mdl.

Not in this PR

• Other schema-typed expression properties (a column's Visible, among others) keep the quoted form until the schema-driven step. The alter-page skill says so.
• mx check was not run. There is no mxbuild in this environment.

Checks

make build, make test, make lint, make check-mdl and make check-findings pass on current main.

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