feat(app): unread message indicator (channel view, header, hamburger)

Tracks per-lane unread counts for finalized assistant messages. A message
counts as unread unless the user is actively reading that lane (current
lane, app focused, newest content at the bottom of the viewport).

- ChatStore: ephemeral per-lane unread map (markUnread/markLaneRead/unreadFor).
- IrisController: 'being read' decision on incoming messages; clears the
  current lane when the app returns to the foreground at the bottom.
- Bridges: push foreground changes to the controller.
- ChatScreen: per-channel badges in the drawer/rail, an 'N new' pill in the
  header (tap glides to the latest message), and a red dot on the hamburger
  (single-pane/mobile only) when another channel has unread.

Closes #3.

app/shared/src/commonTest/kotlin/iris/data/ChatStoreUnreadTest.kt

@@ -0,0 +1,70 @@
+package iris.data
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+/**
+ * M8: per-lane unread tracking (the data behind the channel-view / header
+ * unread indicator). The increment/clear *policy* (which incoming message is
+ * "being read") lives in IrisController; here we cover the store's count
+ * semantics that the UI and controller build on.
+ */
+class ChatStoreUnreadTest {
+    @Test
+    fun markUnreadIncrementsPerLane() {
+        val store = ChatStore()
+        assertEquals(0, store.unreadFor("default"))
+        store.markUnread("default")
+        store.markUnread("default")
+        store.markUnread("chan_7")
+        assertEquals(2, store.unreadFor("default"))
+        assertEquals(1, store.unreadFor("chan_7"))
+        // A lane that never had a message stays at 0.
+        assertEquals(0, store.unreadFor("chan_9"))
+    }
+
+    @Test
+    fun markLaneReadClearsOnlyThatLane() {
+        val store = ChatStore()
+        store.markUnread("default")
+        store.markUnread("chan_7")
+        store.markLaneRead("default")
+        assertEquals(0, store.unreadFor("default"))
+        // Other lanes are untouched.
+        assertEquals(1, store.unreadFor("chan_7"))
+    }
+
+    @Test
+    fun markLaneReadIsIdempotent() {
+        val store = ChatStore()
+        store.markLaneRead("default") // no unread -> no-op
+        assertEquals(0, store.unreadFor("default"))
+        store.markUnread("default")
+        store.markLaneRead("default")
+        store.markLaneRead("default") // clearing twice is safe
+        assertEquals(0, store.unreadFor("default"))
+    }
+
+    @Test
+    fun unreadMapReflectsCounts() {
+        val store = ChatStore()
+        assertNull(store.unread.value["default"])
+        store.markUnread("default")
+        assertEquals(1, store.unread.value["default"])
+        store.markLaneRead("default")
+        // A cleared lane is removed from the map (absent == 0 unread).
+        assertNull(store.unread.value["default"])
+    }
+
+    @Test
+    fun clearWipesUnread() {
+        val store = ChatStore()
+        store.markUnread("default")
+        store.markUnread("chan_7")
+        store.clear()
+        assertEquals(0, store.unreadFor("default"))
+        assertEquals(0, store.unreadFor("chan_7"))
+        assertEquals(emptyMap(), store.unread.value)
+    }
+}