Add opt-in Linux renderer sandbox support to WebContent and WebWorker.
Ladybird and test-web pass --enable-sandbox through when requested, and
the renderer services only install the shared sandbox when that flag is
present.
Share one renderer policy for both services. Allow resource, font,
shared library, WebGL, Wasm, audio, and local IPC paths needed at
runtime, while keeping renderer filesystem writes mediated by Landlock.
Allow Mesa and PulseAudio to probe their standard runtime state without
escaping the renderer sandbox. Return EPERM for scheduler and priority
changes so library initialization can fall back instead of crashing on a
seccomp violation.
