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/codex/translate.rs

@@ -230,6 +230,7 @@ impl Translator {
                         // Cached input is a subset of this figure, not an additional count.
                         context: last.get("inputTokens").and_then(Value::as_u64),
                         tokens_per_second: None,
+                        prefill_ms: None,
                     })
                     .into_iter()
                     .collect()
@@ -247,6 +248,7 @@ impl Translator {
                             tokens: input.unwrap_or(0) + output.unwrap_or(0),
                             context: input,
                             tokens_per_second: None,
+                            prefill_ms: None,
                         });
                     }
                 }
@@ -939,6 +941,7 @@ mod tests {
                 tokens: 18,
                 context: Some(13),
                 tokens_per_second: None,
+                prefill_ms: None,
             }
         );
         assert!(translator.completed());
@@ -1162,6 +1165,7 @@ mod tests {
                 tokens: 42,
                 context: Some(39),
                 tokens_per_second: None,
+                prefill_ms: None,
             }]
         );
         assert_eq!(
@@ -1172,6 +1176,7 @@ mod tests {
                 tokens: 65,
                 context: Some(61),
                 tokens_per_second: None,
+                prefill_ms: None,
             }]
         );
         assert_eq!(