[codex] Hide vendored C symbols in NIF

[Valian]

Summary

• build vendored Lexbor, QuickJS, and WAMR C code with hidden default symbol visibility
• stop patched QuickJS-NG headers from forcing default visibility unless QuickJS is explicitly built as a shared library/module
• keep the intended NIF and N-API exports visible for BEAM loading and native addon support

Root Cause

On macOS ARM64, the precompiled QuickBEAM NIF exported vendored parser/runtime symbols such as Lexbor CSS selector symbols into the process. If QuickBEAM loaded before LazyHTML, LazyHTML could bind against QuickBEAM's already-loaded Lexbor symbols and its CSS selector parser failed permanently.

Fixes #10

Validation

• git diff --check -- lib/quickbeam/native.ex priv/c_src/quickjs.h
• rebuilt with QUICKBEAM_BUILD=1 mix compile --force
• verified nm -gU no longer exposes lexbor_*, lxb_*, JS_*, js_*, wasm_*, or bh_* vendored C symbols
• reproduced load order in a temporary project with QuickBEAM loaded before LazyHTML; LazyHTML.query("[class=x]") now returns one node
• QUICKBEAM_BUILD=1 mix test test/quickbeam_test.exs