RTE.editor in page TSconfig is undocumented — need Core input on the exact behaviour

Splitting this out of #1565, whose second half asks for RTE.editor examples in the Page TSconfig chapter. Writing them turned out to need answers only a Core developer can give, so this is a request for help rather than a ready-to-write task.

The gap

The rte_ckeditor Configuration Reference sends readers here:

Page TSconfig — We recommend you to put all configurations for the preset in the YAML configuration. However, it is still possible to override these settings through the page TSconfig. You can find a list of configuration properties in the Page TSconfig reference, chapter RTE.

Our RTE chapter documents disabled, buttons and proc — but not editor, which is where CKEditor itself is configured, nor preset. So the trail ends without an answer, which is exactly the complaint in #1565.

What the source says (checked on main, 14.3 and 13.4)

• Richtext::getConfiguration() loads the preset, then merges the page TSconfig RTE.* tree over it with ArrayUtility::mergeRecursiveWithOverrule(); addFlattenedPageTsConfig() supplies the un-dotted copy, so RTE.default.editor.config.<option> lands on the preset's editor.config.<option>.
• RichTextElement takes richtextConfiguration['editor'] and hands ['config'] to CKEditor via jsonEncodeForHtmlAttribute().
• CKEditor5Migrator then post-processes (on main/14.3 via AfterRichtextConfigurationPreparedEvent, on 13.4 still called inline).

Where source and documentation disagree

rte_ckeditor/Documentation/Configuration/Concepts.rst states:

be aware that boolean values can not be set, and arrays are not merged but overridden.

ArrayUtility::mergeRecursiveWithOverrule() recurses whenever both sides are arrays, i.e. arrays are merged key by key. The two readings give opposite results for the most obvious real-world case — adding a stylesheet:

RTE.default.editor.config.contentsCss.10 = EXT:site_package/Resources/Public/Css/rte.css

Merged → the preset's contents.css and rte.css are loaded. Overridden → only rte.css. We do not want to document the wrong one.

Questions for a Core developer

1. Is overriding editor.config.* from page TSconfig supported, or discouraged in favour of presets? A GitHub code search finds no site package using it — only copies of Concepts.rst.
2. Arrays: merged key by key, or replaced? Which of the two statements above is correct, and should Concepts.rst be corrected?
3. Booleans: confirmed unusable, since page TSconfig yields strings and "0" is truthy in JavaScript? Is there a recommended way to switch an option off?
4. Is __UNSET (supported by mergeRecursiveWithOverrule) intended to be usable here to drop a key from the preset?
5. Should this be documented in the TypoScript reference at all, or should the rte_ckeditor manual own it, with only a link from here?

Happy to write the section as soon as 1–4 are settled; a branch with a first draft exists already.

Start with rte_ckeditor/Documentation/Configuration/Concepts.rst and the Page TSconfig RTE reference, then inspect the named Richtext::getConfiguration(), RichTextElement, and CKEditor5Migrator paths. Confirm the four behavior questions with a Core developer before writing; done means the ownership, merge behavior, boolean handling, and __UNSET support are settled and documented consistently.