[ako]

Closes #1554 (mendixlabs#1622)

What it does

describe page M.P prints a classic data grid, a reference selector, a drop-down or a reference set selector as a -- NOT re-executable comment. Executing that output replaced the page with the widget gone. The only output was Replaced page M.P, with no warning, and mx check was clean on both sides.

The issue offered two remedies: refuse, or keep the widgets. This PR refuses, which is the repo's guard-don't-drop posture (ADR-0005, the same as validate_workflow_rewrite.go). A create or modify / create or replace of a stored page, snippet or layout is refused when the stored document holds such a widget and the statement doesn't declare it by name.

The refusal says what to do next:

page MyFirstModule.Orders holds 2 widget(s) MDL cannot author, and rewriting it from this statement would delete them:
  - grid1 (Forms$DataGrid)
  - referenceSelector1 (Forms$ReferenceSelector)
  `describe page` prints them as `-- NOT re-executable` comments, which is why executing its output drops them.
  To change the page and keep them, use ALTER, which edits only what it names:
    alter page MyFirstModule.Orders { set (Caption: '…') on <widget>; insert after <widget> { … }; replace <widget> with { … }; };
  To rebuild it without them, remove them on purpose first:
    alter page MyFirstModule.Orders { drop grid1, referenceSelector1; };

• check -p reports the same before anything is written. This is the "pre-check" the issue asks for. It runs in statement order and skips a document that an earlier alter, create or drop in the same script touches.
• Where the widget list comes from: describe's own bookkeeping. describeDroppedWidget already ran for exactly these widgets; now it also feeds the guard, which describes the stored document into io.Discard. There is no second list of authorable $Types to drift from the describe switch.
• A widget the statement declares by name is exempt. Otherwise a widget the builder can write but describe cannot print would refuse its own script.
• Every rebuild call site is covered: pages, snippets and layouts. Layout describe now goes through describeWidgetBody; its output is identical when nothing is dropped.

Keeping the widgets in place across a rewrite would need new MDL syntax for "the stored widget here" and a raw-BSON splice that carries selection references. That is a larger proposal, not this fix.

Testing

• make build, make test, make lint (including conformance), make check-findings and make test-integration-roundtrip all pass locally.
• New tests are in mdl/executor/issue1622_page_rewrite_unauthorable_test.go. They run on a real backend against a fixture copy, with a Studio Pro-shaped Forms$DataGrid (the [up#1565] A fall-through error handler on an activity inside an IF branch is wired to the wrong place: an endless loop with a result variable, a silent end without one #1459 shape) and Forms$ReferenceSelector spliced into the stored unit. No fixture project holds these widgets.
  • The page, snippet and layout rewrites are refused and the stored widgets are kept.
  • check -p reports it.
  • alter … drop then rewrite goes through.
  • An earlier alter in the same script is honoured by check.
  • A widget declared by name is not counted.
• Control: I removed the fix and kept the tests. Every guard test then fails with the reported symptom: Replaced page MyFirstModule.Orders, the stored snippet lost its data grid, the stored layout lost its reference selector. The two "goes through" tests pass both ways; they are the controls against over-refusal.
• mdl/roundtrip/dangling_selection_1415_test.go asserted the old behaviour, namely that the description of a page with a legacy grid executes, which is this bug. It now asserts three things: the refusal names the grid, the [up#1533] Two unhelpful parse messages in workflow MDL (reserved activity name close, exposed before returns) #1415 dangling-selection error stays gone, and the stored page keeps its grid.
• make test-integration-executor had one local failure, TestMxCheck_DoctypeScripts (REST/workflow property sets), which this change doesn't touch. I ran it on a clean origin/main worktree with the same mxbuild 11.6.6 and it fails the same way. CI pins 11.12.2.

Mendix validation

This is a refusal on the mxcli side, so nothing new is written. The repro in the issue (Mendix 10.24, MPR v1) isn't available here. No fixture holds these widgets, so they were seeded into stored units.

Agentic Code Testing

• Skill updated (alter-page: a page holding such widgets refuses the rewrite; use alter)
• docs-site create-page.md and alter-layout.md, CHANGELOG, bug-test mdl-examples/bug-tests/1622-rewrite-keeps-unauthorable-widgets.mdl
• The error message names each widget and both ways forward

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