docs(html-editing): add implementation plan
Eight TDD tasks: the view-block scanner and virtual document, the HTML service wrapper, merging HTML into completion and hover, folding and linked editing, auto-close on type, standing down the duplicate client provider, manifest guards, and a manual editor check. Task 1 comes first because everything reads positions through it: its length-and-newline invariant is what removes position mapping, and a break there would misreport positions everywhere rather than fail. The last task is manual verification in an Extension Development Host. Unit tests cannot show that completions actually appear in an editor, and a green suite has hidden non-functional features in this repo before. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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