JST-PG-006: Bundle + CodeMirror playground

Browser playground with runtime.js (output capture, math/string helpers),
worker.js (Web Worker pipeline: parse→lower→infer→codegen→execute),
CodeMirror editor with dark theme, Run button, examples dropdown,
and build_playground() Julia function for assembling the bundle.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Dale-Black