chore: verify the modernized package and documentation release

[codeacme17]

What to build

Prove that the completed modernization is releasable as one coherent Echo UI version. Verification must exercise the packed artifact as a real consumer would, across supported React and Tailwind generations, while also validating the Nextra site and browser audio behavior.

Acceptance criteria

• Frozen installation, lint, type checks, tests, library build, example build, and Nextra production build pass on the declared runtime baseline.
• A packed artifact installs and renders in React 18.3 and React 19 consumer fixtures without bundling a second React copy.
• The documented Tailwind 3 and Tailwind 4 installation paths are executed against the package versions claimed by the compatibility table.
• Package exports, declarations, styles, bundle contents, external dependencies, and side-effect metadata are audited.
• Nextra route/link checks and representative browser audio tests pass against production-like builds.
• Deprecated IslandJS/NextUI tooling, unintended duplicate dependencies, stale generated output, and undocumented compatibility bridges are absent.
• Release notes clearly describe supported versions and required React, Tailwind, documentation, and Tone migration steps.

Blocked by

• chore: clean dependency boundaries and update low-risk packages #76 — chore: clean dependency boundaries and update low-risk packages.
• build: migrate type checking and linting to TypeScript 6 and ESLint 10 #77 — build: migrate type checking and linting to TypeScript 6 and ESLint 10.
• build: upgrade the library build and test stack to Vite 8 and Vitest 4 #78 — build: upgrade the library build and test stack to Vite 8 and Vitest 4.
• docs: cut over to Nextra and remove IslandJS #84 — docs: cut over to Nextra and remove IslandJS.
• feat: support React 19 while preserving React 18 consumers #85 — feat: support React 19 while preserving React 18 consumers.
• style: support and document Tailwind CSS 3 and 4 installation paths #86 — style: support and document Tailwind CSS 3 and 4 installation paths.
• feat: migrate audio hooks and examples to Tone.js 15 #87 — feat: migrate audio hooks and examples to Tone.js 15.