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/main/kotlin/com/example/aiapp/TranscriptItems.kt

@@ -80,6 +80,8 @@ sealed class TranscriptItem {
          * known until the reply is over.
          */
         val tokensPerSecond: Double? = null,
+        /** How long the provider spent reading the prompt, where it measured that. */
+        val prefillMs: Long? = null,
     ) : TranscriptItem()
 
     /**
@@ -633,7 +635,11 @@ fun foldEvent(items: List<TranscriptItem>, entry: SeqEvent): List<TranscriptItem
         is SessionEvent.UsageDelta ->
             when (val last = items.lastOrNull()) {
                 is TranscriptItem.AssistantMsg ->
-                    items.dropLast(1) + last.copy(tokensPerSecond = event.tokensPerSecond)
+                    items.dropLast(1) +
+                        last.copy(
+                            tokensPerSecond = event.tokensPerSecond,
+                            prefillMs = event.prefillMs,
+                        )
                 else -> items
             }
         is SessionEvent.ContextWindow -> items