feat(pages): write a page variable's default as a bare expression

[ako]

What

A page or snippet variable's default is a Mendix expression, but MDL took it in a string whose content was the expression: $show: Boolean = 'true', = 'if (3 < 4) then true else false'. It can now be written bare, like every other expression (R5):

create page M.P (Title: 'P', Layout: Atlas_Core.Atlas_Default,
  Variables: ( $show: Boolean = true, $limit: Integer = 20,
               $big: Boolean = if 3 < 4 then true else false,
               $label: String = 'Price' + ' list', $status: Enumeration(M.Status) = M.Status.Open )) { … };
alter page M.P { add variables $extra: Boolean = not(false) };

This works in create page, create snippet and alter page … add variables, which all share variableDeclaration.

Language version

mdl 1 is frozen (ADR-0011), so = 'true' cannot change meaning in it. The string form therefore keeps its meaning (its content is the expression) under every language version. It becomes the deprecated alias MDL-DEPR086, refused from mdl 2, following the precedent of MDL-DEPR080 for workflow expressions:

• check and exec warn on it.
• fmt --upgrade rewrites it to the bare form.

One exception: a default that is itself a Mendix string ($s: String = '''abc''') or is empty has no bare spelling yet, because a bare 'abc' is this very alias. Those keep their current form and are not reported.

Changes

• Grammar: variableDeclaration takes (STRING_LITERAL /* @alias MDL-DEPR086 */ | expression).
• Visitor: stores the bare expression's source text. ExitVariableDeclaration records DEPR086 with the rewrite. VariableDefaultReadsBack decides whether a default can be written bare.
• Describe: prints the bare default whenever it reads back as the same default, and the quoted form otherwise.
• Registry: DEPR086 added with an example and a canonical example, which the registry tests check. The generated migration table in docs-site/src/language/versions.md is regenerated.
• Example scripts: the five scripts using the quoted form were converted with fmt --upgrade itself, which changed only their defaults (including an enumeration value).
• Docs, syntax help, skills, the MDL-WIDGET34 suggestion and the R2 header-map test fixtures now use the bare form. Those fixtures had the quoted form as their canonical case.

Verification

• Unit tests:
  • every bare form, including a list whose function-argument commas aren't taken for the list's
  • snippet and ALTER
  • DEPR086 recorded only on the alias, under both mdl 0 and mdl 1, and not on '''abc''' or ''
  • the fmt --upgrade rewrite for create and ALTER
  • the describe round trip
• Control: forcing describe back to always quoting fails the round-trip test (3 cases).
• End to end on a copy of ako/TestApp (Mendix 11.14.0):
  • The stored DefaultValue is exactly the expression written for every form; '''abc''' stores 'abc'.
  • describe prints them bare, and only the one quoted default warned.
  • mx check is clean, including a column Visible: $a and $big over the variables.
• Integration test: TestPageVariableInputBinding_RoundTrip passed locally (it ran, not skipped).
• Gates: make build && make test && make lint && make check-mdl && make check-migration-reference && make check-findings all pass on the branch rebased onto main.

Found while verifying, not changed here: Mendix rejects a default that refers to another page variable ($x: Boolean = not($a) is CE0117 in mx check), and the quoted spelling fails the same way. A check rule for it would be a separate change.

🤖 Generated with Claude Code

https://claude.ai/code/session_01P65SqmwwvbWdJVRwhYiMQw

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