feat(editor): migrate editing surface to CodeMirror 6

[charlypa]

Replace CM5 with a CM6 EditorView adapter and Phoenix-owned compatibility facade.

Virtualize historical module, addon, mode, keymap, theme, text, and filesystem imports.

Update core editor consumers, integrated extensions, live preview, Markdown editing, and themes.

Add CM6 bundling, license generation, zero-CM5 validation, release checks, and regression tests.

Harden Phoenix Builder process, build, WebSocket, and test control with automated coverage.

Document architecture, review order, risks, exclusions, reproduction steps, and test evidence.

Validation: Builder matrix 4,811/4,811; conformance 74/74; validator 15/15; MCP 16/16.

Runtime smoke checks cover Chromium, Firefox, Electron, and Tauri source loading.

What kind of change does this PR introduce?

• Bugfix - fixes issues- give issue github url/details here.
• Feature
• Docs
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Screenshots or Gifs of the change

1. Any visual or workflow changes must come with a gif or screenshot.
2. Gif screen recordings help more if present. You can use one of the below free screen recorders:
   1. windows gif recorder- https://www.screentogif.com/
   2. mac gif recorder- https://apps.apple.com/au/app/giphy-capture-the-gif-maker/id668208984?mt=12
   3. linux gif recorder- https://github.com/phw/peek

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Tests done

• Describe unit tests done
• Describe integration tests done
• Describe manual tests done

Checklist

• When resolving issues, reference those issue URLs in description.
• Propose a changelog if this pull request has any customer facing impact.
• I have added a convincing reason for adding this feature, if necessary.

Other information

pull request guidelines:

• It's OK to have multiple small commits as you work on the PR - we will let GitHub automatically squash it before merging.
• If adding new feature:, Provide convincing reason to add this feature. Ideally you should open a suggestion issue first and have it greenlighted before working on it.
• If fixing a bug:Provide detailed description of the bug in the PR, or link to an issue that does.
• Provide screenshots or gifs of the changes whenever possible.

[sonarqubecloud]

Quality Gate failed

Failed conditions
E Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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