Add interactive playground with live TypeScript execution

[Copilot]

The playground pages were static code examples with no ability to edit or run code. This converts them into a full interactive TypeScript playground running entirely in the browser.

Approach

No WASM needed. The TypeScript compiler (typescript@5 from CDN) transpiles user code to JS in-browser, and new Function() executes it against the tsb bundle (already built by CI into playground/dist/).

Changes

• playground/playground-runtime.js — Shared runtime: loads TS compiler + tsb bundle, transforms import { X } from "tsb" → bundle reference, captures console.log output, supports Ctrl+Enter / Tab / Reset
• playground/index-playground.html — Rewritten: 9 editable code blocks with ▶ Run / ↺ Reset, loading overlay, auto-resizing editors, free-form scratch pad
• docs/playground.md — Documents required properties for all playground pages and how to add new ones
• biome.json — Ignores playground/**/*.js (browser code, not library source)

Usage

Each code block is a live editor:

import { Index } from "tsb";

const idx = new Index(["a", "b", "c"], "demo");
console.log(idx.toString());
// → Index([a, b, c], name='demo')

Press ▶ Run or Ctrl+Enter to execute. Output renders below the editor.

Screenshot

[mrjf]

@copilot reference the playground doc in the readme