PC host: review fixes (idle watchdog, controller close race, malformed input, loader path)

- The 10 s watchdog no longer ends idle sessions. PipeWire and WGC deliver
  frames only on damage; fail only when a frame entered the encoder and never
  came out, or the pipeline never produced its initial frame.
- Controller.state/call/update are inert after close(), so a /status poll
  racing Stop cannot dereference the freed native controller.
- Malformed viewer fields drop the event instead of ending the session.
- PATH/LD_LIBRARY_PATH prepends add no empty (current directory) entry.
- portal.c: document fd ownership. pipewiresrc dups the fd it is given
  (gstpipewirecore.c, F_DUPFD_CLOEXEC), so the portal closing its own fd is
  correct; clear it after close.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/frame_pcview.py

@@ -11,6 +11,12 @@ from frame_macview import MacView, MacViewError, ROOT
 from frame_pc_capture import LIBRARY, NATIVE
 
 
+def prepend(env, name, path):
+    """An empty entry means the current directory to the loader; never add one."""
+    rest = env.get(name, '')
+    env[name] = str(path) + (os.pathsep + rest if rest else '')
+
+
 class PCView(MacView):
     viewer_profile = 'pc-view'
     host = 'windows' if sys.platform == 'win32' else 'linux'
@@ -46,9 +52,9 @@ class PCView(MacView):
         env['GST_REGISTRY_1_0'] = os.path.join(cache, 'gstreamer-registry.bin')
         env['GST_REGISTRY_FORK'] = 'no'
         if sys.platform == 'win32':
-            env['PATH'] = str(NATIVE / 'bin') + os.pathsep + env.get('PATH', '')
+            prepend(env, 'PATH', NATIVE / 'bin')
         else:
-            env['LD_LIBRARY_PATH'] = str(NATIVE / 'lib') + os.pathsep + env.get('LD_LIBRARY_PATH', '')
+            prepend(env, 'LD_LIBRARY_PATH', NATIVE / 'lib')
             env['PIPEWIRE_MODULE_DIR'] = str(NATIVE / 'lib' / 'pipewire-0.3')
             env['SPA_PLUGIN_DIR'] = str(NATIVE / 'lib' / 'spa-0.2')
             env['PIPEWIRE_CONFIG_DIR'] = str(NATIVE / 'share' / 'pipewire')