feat(m0): scaffold Carbon Type foundation — React 19, ProseMirror (MIT), Vite 6

[Copilot]

Replaces the previously approved (but rejected) TipTap-based plan with a fully MIT stack using ProseMirror directly. Bootstraps the entire project from a blank repo.

Stack

• React 19 + Vite 6 + TypeScript (strict)
• ProseMirror (10 MIT packages) — no TipTap, no freemium surface
• CSS Modules + CSS custom properties (dark-first, light via prefers-color-scheme)
• Vitest + @testing-library, ESLint 9 flat config, Prettier

Editor layer (src/editor-core/)

• Schema: doc → title block+; block nodes: paragraph, heading (H1–H6), blockquote, code_block, hr, hard_break; marks: em, strong, code, link, underline, strikethrough
• Plugins: history, keymaps (Mod-b/i/z/Shift-z), gapcursor, inputRules for *em* / **strong** / `code`
• Commands: toggleBold/Italic/Code/Underline, setHeading(level), insertHardBreak
• EditorCore: idiomatic useEffect + useRef ProseMirror mount; ARIA on the outer container (role="textbox", aria-multiline, aria-label) to avoid duplication on the inner contenteditable

useEffect(() => {
  const view = new EditorView(containerRef.current!, { state });
  return () => view.destroy();
}, []);

UI (src/components/)

• Toolbar: role="toolbar", native <button> elements, B / I / U / H1–H3
• App: <header> + <main> landmark structure

Infrastructure

• src/styles/tokens.css — centralized design tokens, dark defaults, light overrides
• src/styles/global.css — box-sizing reset, :focus-visible ring, prefers-reduced-motion
• GitHub Actions CI: lint → build → test, contents: read permissions
• 12 ProseMirror schema smoke tests

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