Add headless contenteditable core

[developer-1px]

Summary

• add a DOM-free contenteditable core that owns native input/composition leases, selection sync, clipboard planning, and history dispatch
• reduce the web adapter to DOM observation/restoration, native event binding, and ClipboardEvent I/O
• move structured text-surface fragment helpers out of ClipboardEvent code and export the new core interface
• add Node-environment tests for native input, IME composition, copy/cut/paste fragments, and selection-based paste

Verification

• npm run typecheck -w @interactive-os/json-document-contenteditable-web
• npm test -w @interactive-os/json-document-contenteditable-web
• npm run typecheck -w @interactive-os/json-document-contenteditable-react
• npm test -w @interactive-os/json-document-contenteditable-react
• npm run extensions:check
• npm run docs:generate
• npm run docs:evaluate
• npm run extensions:verify
• git diff --check