feat(android): OutputPickerViewModel - sheet lifecycle + selection

Bluetooth slice (3/5). HiltViewModel projecting the controller's
routesState Flow plus a sheetVisible MutableStateFlow that owns
the sheet's open/close state. onChipTapped + onSheetDismissed
forward to the controller's discovery toggle so active MediaRouter
discovery only runs while the sheet is visible (battery cost).

Compose UI + NowPlaying wiring land next.

android/app/src/main/java/com/fabledsword/minstrel/player/output/OutputPickerViewModel.kt

@@ -0,0 +1,60 @@
+package com.fabledsword.minstrel.player.output
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.stateIn
+import javax.inject.Inject
+
+/**
+ * NowPlaying-scoped projection over [OutputPickerController]. The
+ * controller is the long-lived Hilt singleton (its MediaRouter
+ * callback must not churn with screen lifecycle); this ViewModel is
+ * a thin lens over its [OutputPickerController.routesState] Flow
+ * plus the sheet's visibility state.
+ *
+ * Sheet open/close are forwarded to the controller's discovery
+ * toggle so active MediaRouter discovery only runs while the sheet
+ * is actually visible.
+ */
+@HiltViewModel
+class OutputPickerViewModel @Inject constructor(
+    private val controller: OutputPickerController,
+) : ViewModel() {
+
+    val routes: StateFlow<RouteSnapshot> = controller.routesState
+        .stateIn(
+            scope = viewModelScope,
+            started = SharingStarted.WhileSubscribed(STOP_TIMEOUT_MS),
+            initialValue = controller.routesState.value,
+        )
+
+    private val sheetVisibleInternal = MutableStateFlow(false)
+    val sheetVisible: StateFlow<Boolean> = sheetVisibleInternal.asStateFlow()
+
+    fun onChipTapped() {
+        sheetVisibleInternal.value = true
+        controller.upgradeDiscovery()
+    }
+
+    fun onSheetDismissed() {
+        sheetVisibleInternal.value = false
+        controller.downgradeDiscovery()
+    }
+
+    fun onRouteSelected(route: OutputRoute) {
+        controller.select(route)
+        sheetVisibleInternal.value = false
+        controller.downgradeDiscovery()
+    }
+
+    private companion object {
+        // SharingStarted timeout so quick screen-orientation changes
+        // don't tear down + re-subscribe the controller's Flow.
+        const val STOP_TIMEOUT_MS = 5_000L
+    }
+}