fix: webcam identity->device key for output frames; top bar always has a Start; light-pill grouping

Take two (2026-09-01) three confirmed defects, all from creator feedback + the
new resolver logging:

1. NO WEBCAM IN OUTPUT: WebcamSceneConfig.WebcamId carries the identity GUID;
   CameraManager keys sessions by DEVICE id — GetLatestFrame(guid) returned null
   forever, the compositor silently dropped the layer while the preview (bitmap
   path) looked fine. The resolver logging added in 8dcaee0 caught it red-handed.
   DeviceKeyForWebcam maps identity->device through the _webcam singleton (identity
   mismatch / unknown ids pass through). Test: WebcamOutputKeyTests.

2. START BUTTON VANISHED AFTER STOP: my pills-clear ruling met the old
   ShowPrimaryStartButton gate (needed IsConnected or a lit REC pill) — signed-out,
   pills-off = blank top bar, no session reachable. Start is now ALWAYS the idle
   face; unarmed Start records (local needs no account — no dead-end no-op); the
   Sign In button folds into Start's context menu ('Sign in to YouTube', shown while
   disconnected) with a dedicated SignInCommand. SessionTeardownTests extended:
   stopped session must leave a reachable Start.

3. TOP BAR ORDER (creator spec): [sign light] REC [pill]  [sign light] ON-AIR [pill]
   — each reality lamp now sits in front of its own intent switch (was: two pills,
   then two orphaned dots). Sign labels keep the shared StatusSignText style.

Tests: WebcamOutputKey, SessionTeardown, FramePump, WebcamMenuGate, GlobalHotkey,
BroadcastPullOut — 14/14 across the six classes, clean build 0 warnings.

ytLive.Tests/SessionTeardownTests.cs

@@ -56,6 +56,9 @@ public sealed class SessionTeardownTests
             Assert.False(vm.OnAirPillOn, "both pills clear");
             Assert.False(vm.ShowEndStreamButton, "no End button once nothing is running");
             Assert.Equal(ytLive.Models.StreamStatus.Offline, vm.StreamStatus);
+            // No dead ends (2026-09-01): after a stop the top bar must still offer a
+            // reachable Start — pills cleared + signed out used to blank the button.
+            Assert.True(vm.ShowPrimaryStartButton, "a stopped session must leave a Start button");
         }
         finally
         {