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>

stubs/macros.ts

@@ -0,0 +1,20 @@
+// Global compile-time MACRO constants
+// In the real Bun build, these are injected via --define at compile time.
+// Here we provide runtime values matching the published v2.1.88.
+declare global {
+  const MACRO: {
+    VERSION: string
+    BUILD_TIME: string
+    FEEDBACK_CHANNEL: string
+    ISSUES_EXPLAINER: string
+    ISSUES_EXPLAINER_URL: string
+    FEEDBACK_CHANNEL_URL: string
+    NATIVE_PACKAGE_URL: string | null
+    PACKAGE_URL: string
+    VERSION_CHANGELOG: string
+  }
+}
+
+// This is never actually executed — the global is set in the entrypoint wrapper.
+// But we need it so TypeScript doesn't complain about `MACRO` being undeclared.
+export {}