build: use managed QuickJS source for vjsx

[guweigang]

Summary

• exclude test-only helpers from production staging and remove stale unused symbols
• compile embedded vjsx support against a managed QuickJS source checkout with -d build_quickjs
• update dependency scripts, doctor checks, and README guidance away from bundled qjs_*.a archives

Why

Newer V builds emit stricter unused symbol notices, and the production staging path was copying test helper files into the build input. At the same time, vnew -cc tcc cannot reliably link the bundled QuickJS archive on macOS arm64, while building QuickJS from source works for both tcc and cc.

Validation

• make build V_CC=cc VPHP_V_GC=boehm WITH_DB=1
• make prod V_CC=cc VPHP_V_GC=boehm WITH_DB=1
• v -cc cc test src/state_store_test.v
• v -cc cc test src/server_logic_test.v
• v -cc cc test src/openai_runtime_test.v
• v -cc cc test src/inproc_vjsx_executor_test.v
• ./scripts/doctor.sh
• manual vnew -cc tcc -d build_quickjs ... -prod -nocache build with VJS_QUICKJS_PATH=/Users/guweigang/Source/quickjs
• manual vnew -cc cc -d build_quickjs ... -prod -nocache build with VJS_QUICKJS_PATH=/Users/guweigang/Source/quickjs