docs: publish Layman controlled-view migration and release contract

[ddebowczyk]

Summary

• document the stable 0.4.0 controlled-view, controller, inspection, toolbar, DnD, theme, snapshot, and Tauri APIs
• add an explicit v0.3-to-v0.4 migration path with no compatibility surface
• add release notes, a deliberate 0.4.0 version bump, packed-file checks, and an emitted-declaration consumer fixture

Verification

• npm ci
• npm test
• npm run lint
• npm run build:lib
• npm run typecheck:consumer
• npm run typecheck:tauri-example
• npm pack --dry-run
• npm run check:package
• markdownlint README.md CHANGELOG.md docs/guides/migrate-to-controlled-view.md docs/reference/public-api.md docs/releases/0.4.0.md docs/integrations/tauri.md
• git diff --check

Native acceptance

• no Tauri CLI or consuming Tauri application is available in this repository; the release notes require that app-level scenario before npm publication