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/Events.kt

@@ -185,6 +185,12 @@ sealed class SessionEvent {
          * over includes the network and whatever the server was doing between tokens.
          */
         val tokensPerSecond: Double? = null,
+        /**
+         * How long the provider spent reading the prompt before it began answering; null where
+         * nothing measured it. The same rule as [tokensPerSecond]: the provider's own figure, or
+         * nothing at all.
+         */
+        val prefillMs: Long? = null,
     ) : SessionEvent()
 
     /** How much context this session's model has, which is what [UsageDelta.context] is out of. */
@@ -328,6 +334,7 @@ fun parseSeqEvent(json: String): SeqEvent {
                     body.getLong("tokens"),
                     if (body.has("context")) body.getLong("context") else null,
                     if (body.has("tokensPerSecond")) body.getDouble("tokensPerSecond") else null,
+                    if (body.has("prefillMs")) body.getLong("prefillMs") else null,
                 )
             "compacted" ->
                 SessionEvent.Compacted(