Report what a reply spent reading its prompt, and pin the clock right

`UsageDelta` gains `prefillMs`, llama-server's own `timings.prompt_ms`, so the
footer under a finished reply is "read 9.5s · 50.3 tok/s · 3:00 PM". Prefill is
the half of a turn that was invisible and is often the larger: measured on the
0.6B here, 1m 4s for the first turn after a model loads against 22ms for the
next, whose prompt the server still had cached.

The clock moves to the end of the line. Everything in front of it is a
provider's own measurement, so a session on another provider has fewer of them
or none, and a reader who has learned where the time is should not have to find
it again because the model changed. The costs grow leftwards into the space
instead, and a test asserts every shape of the line ends with the same thing.

Verified on the emulator against a real llama session: three replies reading
"read 1m 4s · 193 tok/s · 3:54 PM", "read 25ms · 308 tok/s · 3:54 PM" and
"read 22ms · 194 tok/s · 3:54 PM", with the clock in one column.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/androidApp/src/test/kotlin/com/example/aiapp/ThinkingTest.kt

@@ -14,6 +14,7 @@ import kotlin.test.assertTrue
  * generation speed has no figure -- neither may borrow one.
  */
 class ThinkingTest {
+    private val utc = ZoneId.of("UTC")
     private var seq = 0L
 
     private fun fold(items: List<TranscriptItem>, event: SessionEvent, ts: Double = 1.0) =
@@ -77,29 +78,41 @@ class ThinkingTest {
     }
 
     @Test
-    fun `a reply carries when it was sent and what it was generated at`() {
+    fun `a reply carries when it was sent and what it cost to produce`() {
         val items =
             fold(emptyList(), SessionEvent.AssistantText("Done."), ts = 1_788_609_600.0).let {
-                fold(it, SessionEvent.UsageDelta(42, 100, 18.37))
+                fold(it, SessionEvent.UsageDelta(42, 100, 18.37, 9_489))
             }
         val reply = items.filterIsInstance<TranscriptItem.AssistantMsg>().single()
         assertEquals(1_788_609_600.0, reply.ts)
         assertEquals(18.37, reply.tokensPerSecond)
+        assertEquals(9_489, reply.prefillMs)
 
-        val footer = replyFooterText(reply.ts, reply.tokensPerSecond, ZoneId.of("UTC"))
+        val footer = replyFooterText(reply.ts, reply.tokensPerSecond, reply.prefillMs, utc)
         // The clock reading rather than the whole string: the platform's own short-time format
         // differs by JDK and locale, which is the point of asking it for one.
-        assertTrue(footer!!.contains("12:00"), footer)
-        assertTrue(footer.endsWith("18.4 tok/s"), footer)
+        assertTrue(footer!!.startsWith("read 9.5s · 18.4 tok/s · "), footer)
+        assertTrue(footer.contains("12:00"), footer)
     }
 
     @Test
-    fun `a provider that measures no speed gets a footer of the time alone`() {
-        val footer = replyFooterText(1_788_609_600.0, null, ZoneId.of("UTC"))
-        assertTrue(footer!!.contains("12:00"), footer)
-        assertTrue(!footer.contains("tok/s"), footer)
-        // And a reply with neither has no line at all rather than an empty one.
-        assertNull(replyFooterText(0.0, null, ZoneId.of("UTC")))
+    fun `the clock stays at the end however much the provider measured`() {
+        // What a provider that measures nothing leaves: the time, and nothing in front of it.
+        val bare = replyFooterText(1_788_609_600.0, null, null, utc)
+        assertTrue(bare!!.contains("12:00"), bare)
+        assertTrue(!bare.contains("tok/s") && !bare.contains("read"), bare)
+        // Every shape ends with the same thing, which is the whole point of the order: the clock
+        // does not move because the session is on a provider that measures more or less.
+        val shapes =
+            listOf(
+                bare,
+                replyFooterText(1_788_609_600.0, 18.37, null, utc)!!,
+                replyFooterText(1_788_609_600.0, null, 9_489, utc)!!,
+                replyFooterText(1_788_609_600.0, 18.37, 9_489, utc)!!,
+            )
+        assertEquals(1, shapes.map { it.substringAfterLast("· ") }.distinct().size, "$shapes")
+        // A reply with nothing to say has no line at all rather than an empty one.
+        assertNull(replyFooterText(0.0, null, null, utc))
     }
 
     @Test
@@ -130,7 +143,7 @@ class ThinkingTest {
                 SessionEvent.AssistantText("Reading it."),
                 SessionEvent.ToolStart("t1", "Read", "{}"),
                 SessionEvent.ToolEnd("t1", "done"),
-                SessionEvent.UsageDelta(42, 100, 18.0),
+                SessionEvent.UsageDelta(42, 100, 18.0, 500),
             )
         assertNull(items.filterIsInstance<TranscriptItem.AssistantMsg>().single().tokensPerSecond)
     }