feat(macOS): Restore macOS support

[iFleey]

Hey again! The macOS support from #664 shipped in v3.0.0, but it quietly rotted on main. With no macOS CI to catch it, the async-ffi / sdk-flatten / pipe-CDP refactors reintroduced a handful of Linux-isms and left the CDP transport a #else stub — so on main today it doesn't build, and even patched locally the menus won't open. This PR gets it green again.

Roughly whats in here:

• Build drift
  a few Linux-only assumptions (endian headers, inotify, SOCK_CLOEXEC, a Rust return type) crept back in without __APPLE__ branches. Restored them.

• CDP transport
  this was the real "menus won't open" cause. After the move to a pipe-based transport, macOS was left a stub with its old WebSocket path deleted. macOS is different: the steam_osx wrapper enables CEF remote debugging over TCP, so I implemented the macOS path to talk CDP over a libcurl WebSocket (re-resolving the debugger URL each attempt so a steamwebhelper restart doesn't strand it). First clean run: menus open, Connected to SharedJSContext in 25 ms.

• Shutdown crash
  remember the mutex lock failed: Invalid argument on shutdown from Feature/macOS support based on async-ffi branch #664? macOS only now runs to a clean exit(), which exposed one root cause: a static destruction-order issue where teardown locked singletons __cxa_finalize had already destroyed. Leaking them keeps the sync primitives alive past teardown. Shared code, so it helps every platform.

• Scripts
  scripts/macos/* paths drifted after the layout changes; fixed those too.

I also built a macOS CI job + bundle-based install on a branch stacked on this one — macos-dist. Since it's based on this PR I'm holding off on a second PR, but it's fully green if you want to peek: Github Actions, build-macos passes; the only red job is Windows code-signing, which needs a repo secret my fork doesn't have.

Tested Devices

• MacBook Air (M4, Apple Silicon)

I only have Apple Silicon on hand this round,so it's arm64-only and I couldn't test x86_64. Steam runs arm64 natively onApple Silicon so this is the right slice here.

Thanks for taking a look!

BTW, actually the Extendium plugin does not support the Mac platform.

[45:48.483] EXTENDIUM ========================================
[45:48.483] EXTENDIUM [install] Starting extension installation...
[45:48.483] EXTENDIUM ========================================
[45:48.483] EXTENDIUM [install] Extension directory: ~/Library/Application Support/Millennium/plugins/extendium/fake-header-extension
[45:48.483] EXTENDIUM [install] Reading extension-keys.json: ~/Library/Application Support/Millennium/plugins/extendium/fake-header-extension/extension-keys.json
[45:48.483] EXTENDIUM [install] Extension ID: ilceigfjdfkplpmafppofppbifclfdop
[45:48.484] EXTENDIUM [install] Reading manifest.json: ~/Library/Application Support/Millennium/plugins/extendium/fake-header-extension/manifest.json
[45:48.484] EXTENDIUM [install] Extension: Extendium - Fake Header Extension v1.0.0
[45:48.484] EXTENDIUM [install] Steam path: ~/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS
[45:48.484] EXTENDIUM [install] Platform: Linux
[45:48.484] EXTENDIUM [install] Steam config directory: ~/.steam/steam/config/htmlcache/Default
[45:48.484] EXTENDIUM [install] Steam config directory does not exist: ~/.steam/steam/config/htmlcache/Default