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>

docs/architecture/embedded-mpv-native.md

@@ -202,17 +202,22 @@ Trade-offs and constraints:
 - Scope: on macOS Apple Silicon only by owner decision (2026-07-10);
   Intel Macs keep the native-view engine. Linux (any arch) is ported —
   headless EGL, works under native Wayland since nothing embeds into a
-  window; dev builds need `libmpv-dev`, `libegl-dev`, `libgl-dev` and
-  `libgbm-dev` (the helper links system libmpv, which is legal
-  out-of-process — the in-process libmpv ban still binds the addon). The
-  Windows port of the helper (WGL) is future work — the shm protocol and
-  adapter are platform-agnostic.
+  window; dev builds need `libmpv-dev`, `libegl-dev`, `libgl-dev`,
+  `libopengl-dev` and `libgbm-dev` (the helper links system libmpv, which
+  is legal out-of-process — the in-process libmpv ban still binds the
+  addon). The helper logs the chosen EGL display tier and the GL renderer
+  string to stderr; on systems whose hardware driver is only reachable
+  via the default display (e.g. NVIDIA proprietary), the surfaceless tier
+  can select Mesa's software renderer — check that log line when
+  diagnosing performance. The Windows port of the helper (WGL) is future
+  work — the shm protocol and adapter are platform-agnostic.
 - Measured baseline (M1 Pro, spikes/mpv-frame-copy/RESULTS.md): 4K60 HEVC
   sustained end to end, ~1.2 ms shm copy + ~3.5 ms texture upload, ~10 ms
   produce-to-upload latency, zero torn frames over a 10-minute run.
   Linux (i7-1165G7/Iris Xe, same RESULTS.md): 1080p60 sustained with
-  ~1.2 ms copies; 4K rows are software-decode-limited on that hardware;
-  zero torn frames everywhere.
+  ~1.2 ms copies; the 4K rows are limited by software decode/source
+  generation on that hardware, not by the copy path; zero torn frames
+  everywhere.
 - Helper crash isolation: an unexpected helper exit surfaces as a session
   `error` (renderer falls back); it can never take down the Electron main
   process, unlike in-process libmpv.