MDL repeatability gaps: if-not-exists for add attribute, insert and rename; alter of container actions and documentation; refresh action; check warning for \n #1691

Builders that script a change with MDL need statements that can run twice with the same result. These gaps force one-off scripts or whole-container replacements. Checklist (split into separate issues if preferred):

• alter entity ... add attribute if not exists: a second run of add attribute fails.
• alter page/flow ... insert ... if not exists: a second run fails (duplicate widget name) or is refused. replace <container> is repeatable but copies the neighbouring widgets and can revert later changes.
• alter page cannot set an action on a container, and cannot edit the documentation of a snippet or an attribute (related: alter page cannot change the on-click action of a container: set (Action: …) says "has no Action property", OnClick points back to Action #1592, closed; not re-verified here after that fix).
• A refresh action for pages on a non-persistent data source. Close page + show page is flagged by lint rule FL002; a plain show page re-renders but grows the browser history.
• mxcli check warns on \n in a string literal. In an MDL string literal '\n' is stored as a backslash and an n, not a line break (a plain-text mail showed \n). check and mx check pass; see also check passes expressions the build rejects or that misbehave at runtime (function arguments, Decimal toString, raw backslash literals) #1515 point 3 (raw backslash literals).
• rename microflow|... M.Old to New if exists (no-op when the source is absent or the target exists). A script that renames a flow and creates a thin wrapper under the old name renames the wrapper on a second run. Precedent: drop ... if exists (DROP <document> IF EXISTS, so a script that removes a document can run twice #1190).
• An ALTER of a long flow keeps the error handlers' targets. An alter microflow on a long startup flow that adds calls with handlers rejoined the handlers so the lines crossed.

Environment

mxcli 0.28.0 (first seen on 0.27.0), Studio Pro 11.12.4, Windows 11. Found by the AppGen tooling (Ciphix) on a test app.