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>

package.json

@@ -0,0 +1,20 @@
+{
+  "name": "@anthropic-ai/claude-code-source",
+  "version": "2.1.88",
+  "description": "Claude Code v2.1.88 — decompiled source for research",
+  "type": "module",
+  "private": true,
+  "scripts": {
+    "prepare-src": "node scripts/prepare-src.mjs",
+    "build": "npm run prepare-src && node scripts/build.mjs",
+    "check": "npm run prepare-src && tsc --noEmit",
+    "start": "node dist/cli.js"
+  },
+  "engines": {
+    "node": ">=18.0.0"
+  },
+  "devDependencies": {
+    "esbuild": "^0.27.4",
+    "typescript": "^6.0.2"
+  }
+}