Prepare JavaScript SDK 3.0.0 with playlist write support

[wenbinf]

Prepare podcast-api 3.0.0 with playlist write support and generated endpoint documentation. This is a major release because Node.js 22 or newer is now required; older Node runtimes are no longer supported.

• Add createPlaylist, updatePlaylist, addPlaylistItem, deletePlaylistItem, and updatePlaylistItemNotes, preserving existing method names and response envelopes.
• Generate method wrappers, the API contract snapshot, and marked README sections from the monorepo's canonical OpenAPI specification and JavaScript registry. Keep the SDK and its tests independently usable.
• Add PUT support to Node and Workers transports, encode path identifiers, separate query/body parameters, preserve explicitly empty fields, and isolate each client's settings and API key.
• Replace live API tests with mocked contract tests for all 30 methods, request encoding, error responses, empty fields, client isolation, and README coverage.
• Update dependencies and ESLint configuration. Run Node and Workers CI on Node 22 and 24 using Corepack-managed Yarn 3.3.1 and immutable installs.

Validation: all 84 SDK tests (43 Node, 41 Workers), lint, and immutable installation pass locally. The monorepo generator's --check and all 16 generator/documentation tests pass. All four GitHub CI jobs pass on the 3.0.0 release commit: Node.js CI and Cloudflare Workers CI, each on Node 22 and 24. Automated tests do not call production APIs.

Separate manual production smoke tests confirmed playlist creation, adding an episode, setting and clearing notes, and deleting the item.

npm publication is separate. The monorepo registry now targets 3.0.0 for this release and its five new methods; its published-support record remains at 2.0.4 until npm publication is confirmed. Those companion monorepo changes are separate from this PR.