player: disable mppvideodec auto-rank-boost, add R36S_HW_DECODE=1 opt-in

mppvideodec outputs NV12 (hardware format) which GStreamer videoconvert
converts to BGRA in scalar software code — slower than avdec_h264 which
uses libav's NEON-optimised YUV→BGRA path.

Default behaviour: software decode (avdec_h264) at PRIMARY rank.
The MPP plugin is still detected and logged so the user knows it is
installed and operational.

Set R36S_HW_DECODE=1 to re-enable the rank boost once a zero-copy
NV12→SDL_UpdateNVTexture (or similar) upload path is implemented.

src/r36s_dlna_browser/player/gstreamer_backend.py

@@ -60,15 +60,35 @@ def _vpu_device_accessible() -> bool:
 
 def _probe_hw_decoders(gst_module) -> list:
     """
-    Probe for available hardware video decoder GStreamer elements and boost
+    Probe for available hardware video decoder GStreamer elements.
-    their rank so that playbin's internal decodebin prefers them over software
+
-    decoders.  Returns the list of element names that were found and boosted.
+    Hardware decode (mppvideodec / V4L2) is only auto-selected when
-    If no VPU device node is accessible the probe is skipped entirely so that
+    R36S_HW_DECODE=1 is set in the environment.  By default the probe is a
-    a half-initialised hardware element cannot stall the pipeline.
+    no-op: software avdec_h264 (libav, NEON-optimised) remains preferred
+    because mppvideodec outputs NV12 which requires an additional software
+    NV12→BGRA conversion step that outweighs the decode speedup.
+
+    Set R36S_HW_DECODE=1 (e.g. in MatHacks.sh) once a zero-copy NV12 SDL
+    texture upload path is implemented.
+
+    Returns the list of element names whose rank was boosted.
     """
+    if os.environ.get("R36S_HW_DECODE", "0") != "1":
+        # Still probe and log availability so the user knows the plugin is
+        # installed, but do not boost rank.
+        if _vpu_device_accessible():
+            for name in _HW_DECODER_ELEMENTS:
+                if gst_module.ElementFactory.find(name) is not None:
+                    log.info(
+                        "HW decode: %s available (VPU accessible) — not auto-selected. "
+                        "Set R36S_HW_DECODE=1 to enable (experimental).", name
+                    )
+        return []
+
     if not _vpu_device_accessible():
         log.info(
-            "HW decode: no accessible VPU device node — using software decode. "
+            "HW decode: R36S_HW_DECODE=1 but no accessible VPU device node — "
+            "falling back to software decode. "
             "Run deploy/arkos/setup_hw_decode.sh as root to fix permissions."
         )
         return []