Doobie v0.13.3

Fixed (Linux defensive hardening)

A Linux user still reported a white window on v0.13.2 even after the
webkit2gtk-4.1 fix. Deep audit surfaced three additional silent
failure modes JUCE 8 has no host-side log for. All three are now
guarded in WebEditor.cpp before the WebBrowserComponent is
constructed:

• WebKitGTK DMA-BUF renderer. WebKitGTK 2.42+ defaults to a
  DMA-BUF/EGL path that fails silently (white window, no log) on
  NVIDIA proprietary drivers under Wayland, in VMs without virgl,
  and in several sandbox configs (WebKit bug 262607). Now we
  pre-set WEBKIT_DISABLE_DMABUF_RENDERER=1,
  WEBKIT_DISABLE_COMPOSITING_MODE=1 and GDK_BACKEND=x11 defensively
  (only when those vars aren't already set, so power users can still
  override).
• /tmp mounted noexec. JUCE extracts a WebKit helper binary to
  $TMPDIR/_juce_linux_subprocess* and execvs it; on hardened
  RHEL/Rocky, Snap-packaged DAWs (AppArmor), Fedora 41+ (SELinux),
  and Flatpak/bubblewrap-confined hosts the exec silently fails →
  no WebKit process → blank window. We now probe with a tiny shell
  script, and if exec is denied, relocate $TMPDIR to
  $XDG_RUNTIME_DIR (always exec-allowed under systemd) or
  ~/.cache/doobie before JUCE looks at it. Either way a stderr
  warning is logged so support reports become actionable.
• CI matrix now covers Ubuntu 24.04 alongside 22.04 — modern
  distros catch webkit2gtk-4.1 regressions before users do.

Documentation

• README + CHANGELOG both reference the new env-var workarounds
  users on weird configs can fall back to.