refactor!: converge on the stable JSON editing protocol

[developer-1px]

Summary

• Replace the unreleased editor surface with the single root runtime export mountEditor.
• Make the caller-owned JSONDocument the sole canonical writable value behind a schema-neutral EditorAdapter boundary.
• Normalize W3C Input Events, target ranges, Selection API state, Clipboard Events, and IME composition into atomic JSON edit plans.
• Keep document schemas, history policy, migrations, product commands, ARIA policy, and collaboration semantics outside the root engine.
• Provide source-preserving Markdown as an optional reference adapter at @interactive-os/editable/markdown.
• Remove the legacy schema/session/history and compatibility surface instead of preserving an unreleased API.

Dependency

Depends on json-document PR #238 and its @interactive-os/json-document@2.0.0-rc.0 contract. This branch vendors the package built from source commit 23b1fd7e until registry publication.

• Vendored JSON kernel SHA-256: cfc7403ccb8ec1646b24aa16eed6774e3c77f7fd10766034a0cb70559d919032

Verification

• Internal unit tests: 156/156
• Packed-package tests: 120/120, including clean-install smoke
• Playwright: 27/27 across Chromium, Firefox, and WebKit
• TypeScript, Biome, layer-boundary checks, production builds, package build, and package smoke
• Reproducible editable tarball SHA-256: c251dbaabffae6fad4dc411e07d00c01d56dac3aef33be8b7ed69318394a7f9e

Release conditions still open

• A license must be selected before public distribution.
• Issue 선택 인식 Markdown Live Preview — source-preserving notation reveal 엔진 구현 #143 still has 0/2 required real-OS Korean IME traces: desktop and mobile.
• Automated composition coverage passes, but this PR does not claim real-device IME acceptance or close 선택 인식 Markdown Live Preview — source-preserving notation reveal 엔진 구현 #143.