Add init.js alias so unpkg.com/playhtml/init.js works as a drop-in

[spencerc99]

Problem

The docs (and the natural user expectation) reference a drop-in script at https://unpkg.com/playhtml/init.js, but that file has never been published. The build emits dist/init.es.js (a 2-line module that imports playhtml and calls init({})), and the package files/exports only expose dist/. So the short URL 404s.

PR #220 worked around this by pointing the docs at the verbose https://unpkg.com/playhtml/dist/init.es.js. This issue is the proper fix: make the friendly short URL resolve.

Goal

https://unpkg.com/playhtml/init.js should load and run playhtml.init({}), so the Path A drop-in in getting-started.mdx can use the short, memorable URL.

Approach

unpkg resolves real files on disk in the published tarball (it does not consult bundler-style exports subpath maps for bare paths like /playhtml/init.js). So the fix needs an actual init.js file in the published package root, plus an exports entry for correctness when imported via a bundler.

Concretely:

1. Emit (or add) a root-level init.js in the published package that does the same thing as dist/init.es.js (import { playhtml } from "./dist/playhtml.es.js"; playhtml.init({});). Either change the vite build fileName to also output init.js at the publish root, or add a tiny committed init.js re-export and include it in package.json files.
2. Add "./init.js": "./init.js" (or ./dist/init.es.js) to the exports map.
3. Add init.js to package.json files if it's a committed file.
4. Verify against a real unpkg URL after publish (the build hash path resolves) — curl -sI https://unpkg.com/playhtml@<next-version>/init.js should 200.

Acceptance criteria

• https://unpkg.com/playhtml/init.js returns the init module (200, not 404) after the next release.
• Changeset added (playhtml patch).
• After release, optionally simplify apps/docs/src/content/docs/getting-started.mdx Path A back to https://unpkg.com/playhtml/init.js (and the two prose mentions).
• No regression to the existing dist/init.es.js path.

Context

• Build config: packages/playhtml/vite.config.ts (the fileName fn emits init.${format}.js for the init entry).
• packages/playhtml/package.json files / exports.
• Discovered while auditing docs examples in PR Standardize element-reference resolution (fix can-duplicate with #/selector) #220.