fix(embedded-mpv): address multi-agent review findings on the Linux port

Confirmed findings (each verified by 3 adversarial reviewers):

- CI would fail to link the helper: -lOpenGL needs the unversioned glvnd
  libOpenGL.so, shipped only by libopengl-dev, which neither the runner
  images nor the previous apt line provide. Added to the workflow and to
  every documented Linux build-dep list.
- The new 'test -x' dist guard could never pass: webpack's dist asset
  copy drops file modes (helper arrives as 0644). The guard is now
  'test -f'; electron-after-pack.cjs restores the execute bit on packaged
  helpers (also fixes packaged-macOS spawns); the support probe now
  requires X_OK, so a mode-stripped helper reads as frame-copy-unavailable
  and falls back to native instead of failing spawn with EACCES.
- The Settings frame-copy toggle was unreachable in exactly the Linux
  states the port targets: the native-Wayland and missing-system-mpv
  unsupported payloads omitted frameCopyAvailable, and toggle visibility
  derives solely from it. Both returns now advertise availability.

Also from review:

- build-embedded-mpv.js keeps the old graceful-skip contract when the new
  system-dev fallback finds libmpv-dev but the GL/EGL/gbm dev stack is
  missing (previously such machines skipped; a hard electron-build
  failure was a regression).
- createSession derives the window-handle skip from the dispatched addon
  instead of re-evaluating the engine gate, so the two cannot disagree.
- The render thread logs the GL renderer string (surfaceless Mesa can
  silently pick llvmpipe on non-Mesa-primary systems; now diagnosable —
  verified 'Mesa Intel Iris Xe' on this machine).
- Specs pin the new semantics: frameCopyAvailable advertised while native
  is unsupported (Wayland / no mpv), frame-copy supported without a
  system mpv, and the handle-skip test disposes its session through the
  owning adapter.
- Docs: PORTING.md file map reflects the frame_helper_gl.h seam for the
  Windows porter; helper-strip removal correctly gated on milestone 4
  (bundled libmpv), not milestone 3; RESULTS.md preamble notes the
  RAW->MONOTONIC clock change; stale '(macOS)' scope comments updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/build-and-make.yaml

@@ -68,7 +68,7 @@ jobs:
               if: matrix.os == 'linux'
               run: |
                   sudo apt-get update
-                  sudo apt-get install --no-install-recommends -y rpm libarchive-tools flatpak flatpak-builder appstream libx11-dev libxext-dev libmpv-dev mpv pkg-config libegl-dev libgl-dev libgbm-dev
+                  sudo apt-get install --no-install-recommends -y rpm libarchive-tools flatpak flatpak-builder appstream libx11-dev libxext-dev libmpv-dev mpv pkg-config libegl-dev libgl-dev libopengl-dev libgbm-dev
 
                   # Configure Flatpak
                   # 1. Add the Flathub repository (source of runtimes)
@@ -339,7 +339,10 @@ jobs:
                       # The frame-copy helper is the inverse: a separate process
                       # that MUST link libmpv (dev-mode engine; stripped from
                       # packages until the bundled-runtime staging lands).
-                      test -x dist/apps/electron-backend/native/iptvnator_mpv_helper
+                      # test -f, not -x: the webpack dist asset copy drops file
+                      # modes; consumers restore the bit (after-pack) or require
+                      # it via the X_OK support probe.
+                      test -f dist/apps/electron-backend/native/iptvnator_mpv_helper
                       if ! ldd dist/apps/electron-backend/native/iptvnator_mpv_helper | grep -q 'libmpv'; then
                           echo "::error::Linux frame-copy helper must link libmpv"
                           ldd dist/apps/electron-backend/native/iptvnator_mpv_helper