Add build infrastructure and QUICKSTART guide

- Build script (scripts/build.mjs) using esbuild with iterative stub creation
- Stubs for Bun compile-time intrinsics (feature(), MACRO, bun:bundle)
- Stub modules for feature-gated internal code paths
- QUICKSTART.md with 3 build options (pre-built, esbuild, Bun)
- tsconfig.json, package.json for build tooling

Build reaches ~95% — 108 remaining feature-gated modules need Bun runtime
for full dead code elimination. See QUICKSTART.md for details.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -745,6 +745,8 @@ This source is **not directly compilable** from this repo alone:
 - The compiled `cli.js` is a self-contained 12MB bundle requiring only Node.js >= 18
 - Source maps (`cli.js.map`, 60MB) map back to these source files for debugging
 
+**See [QUICKSTART.md](QUICKSTART.md) for build instructions and workarounds.**
+
 ---
 
 ## License