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>

AGENTS.md

@@ -67,9 +67,11 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   the span the *driver* measured, and the phone draws a card that spins while
   the block is open and says "Thought for 12.4s" once it is not. The reasoning
   is deliberately not part of the next prompt (`conversation` ignores it), and
-  `timings.predicted_per_second` off the same stream becomes `UsageDelta`'s
-  `tokensPerSecond`, which is the "149 tok/s" under a finished reply — nothing
-  else here measures one, so every other driver sends `None`.
+  `timings.predicted_per_second` and `timings.prompt_ms` off the same stream
+  become `UsageDelta`'s `tokensPerSecond` and `prefillMs`, which is the
+  "read 9.5s · 50.3 tok/s · 3:00 PM" under a finished reply — nothing else here
+  measures either, so every other driver sends `None`, and the clock is last so
+  that it does not move when a provider reports fewer of them.
   **A turn's wait has two halves and says which** (2026-09-19):
   `SessionStatus::Loading` is the model coming off disk and
   `SessionStatus::Reading` is `llama-server` processing the prompt -- emitted