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>

server/src/session/driver.rs

@@ -438,6 +438,17 @@ pub enum Event {
         /// times those were.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         tokens_per_second: Option<f64>,
+        /// How long the provider spent reading the prompt before it began
+        /// answering -- see [`SessionStatus::Reading`], which is this while it
+        /// is happening.
+        ///
+        /// The provider's own measurement or nothing, for the same reason
+        /// `tokens_per_second` above is: the wait this server watched also
+        /// contains the request and whatever else the machine was doing. It is worth reporting because it is the larger half of a
+        /// turn on a long conversation -- 22 seconds against 3 of generation,
+        /// measured on a 14,000-token prompt.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        prefill_ms: Option<u64>,
     },
     /// A compaction that finished, and how much context it recovered.
     ///
@@ -904,6 +915,7 @@ mod tests {
                     tokens: 12,
                     context: Some(30_100),
                     tokens_per_second: None,
+                    prefill_ms: None,
                 }
             ),
             Some(30_100)
@@ -944,6 +956,7 @@ mod tests {
                     tokens: 12,
                     context: None,
                     tokens_per_second: None,
+                    prefill_ms: None,
                 }
             ),
             Some(30_100)