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.d.ts

@@ -0,0 +1,15 @@
+/**
+ * Compile-time macros injected by Bun's bundler.
+ * These are replaced with string literals during bundling.
+ * For our source build, we provide runtime values.
+ */
+
+declare const MACRO: {
+  VERSION: string
+  BUILD_TIME: string
+  FEEDBACK_CHANNEL: string
+  ISSUES_EXPLAINER: string
+  NATIVE_PACKAGE_URL: string
+  PACKAGE_URL: string
+  VERSION_CHANGELOG: string
+}