refactor(editor): rebuild diff viewer around code editor kernels

[bzy-debug]

Summary

• replace the public-Viewer-based DiffViewer coordinator with a first-class DiffEditorWidget that owns two code-only CodeEditorWidget kernels and one shared DiffEditorViewModel
• split rich Markdown into an independent MarkdownViewer surface and make desktop/embedded hosts choose code, Markdown, or diff presentation explicitly
• introduce the DocumentDiffProvider contract, keep Core as the default provider, expose MoonDiff as optional host-owned Token/Tree modes, and remove fake moves/timeout compatibility
• make diff updates generation-fenced and transactional across model pairs, decorations, managed ViewZones, viewport restore, overview, and disposal
• virtualize Inline deleted content with one shared render context and a global DOM budget; add responsive Inline/SideBySide layout, sash, overview, F7 navigation, live status, and pane ARIA
• remove the legacy DiffViewer/Unified implementations and add an architecture gate preventing DiffEditor from depending on the public Viewer surface

Breaking changes

• DiffViewer is replaced by DiffEditor; Unified terminology is replaced by Inline
• public Viewer is code-only; Markdown routing is host-owned through MarkdownViewer
• no compatibility aliases or adapters are retained

Validation

• just check
• just test — native 3182, JS 3043, cram 27
• just build
• just editor-test — wasm 1058, JS 1798, native 1186
• just editor-test-browser — 86 passed
• desktop frontend tests — 422 passed
• packaged and codesign-verified the real SeekMoon.app
• real Proton/CEF QA covered the 900/901 responsive boundary, sash 10–90%, Core/Token/Tree fallback, Ignore Comments, F7/Shift+F7, overview pointer, Markdown routing, and Markdown diff code-only rendering; no page or console errors

Rebased onto main at 84bfd310 before publication.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 943df0d089

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".