Say which half of the wait a llama turn is in

A turn has two waits in front of the first token and they were one word.
`SessionStatus::Loading` was already the model coming off disk; this adds
`SessionStatus::Reading` for llama-server processing the prompt -- emitted when
the request goes out, cleared by the first thing the model says of any kind, so
it covers every generate in a tool loop rather than only the first.

Prefill is the expensive half on this machine: measured 9.5s for 6,068 tokens
and 22s for 14,068 on the 27B with the GPU to itself. Reported as `running`
that was indistinguishable from a model thinking, which is the thing the reader
is waiting for. The phone draws both with the working spinner and its own
words -- "loading model" and "reading prompt" -- and the session screen's
status row now spins for all three busy states instead of only `running`,
which is also how `loading` stops being a bare word with nothing moving.

Measured while checking the tok/s figure, and recorded in the rigs skill: the
27B holds 55.5 to 50.3 tok/s between 1.5k and 14k of context, so decode decays
gently, while the 0.6B on the CPU falls 30.1 to 11.5 over 6k. A shared GPU is a
different failure -- the model does not load at all.

Verified on the emulator against a real llama session: "loading model" while
the server started, then "reading prompt" with the spinner through prompt
processing, then the thinking card.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/src/session/driver.rs

@@ -658,6 +658,20 @@ pub enum SessionStatus {
     /// state -- a driver that reports it is responsible for holding what it
     /// is sent until it can deliver it -- but this is what says so on screen.
     Loading,
+    /// The model is reading what it was given, and has not begun answering.
+    ///
+    /// Its own state for the same reason [`SessionStatus::Loading`] is, one
+    /// level down: prompt processing is work the machine does before a turn
+    /// produces anything, and on a long conversation it is the part of the
+    /// wait somebody is looking at. Reported as `Running` it was
+    /// indistinguishable from a model thinking, which is what the reader is
+    /// actually waiting to see.
+    ///
+    /// It is still a turn in progress -- nothing settles, nothing is invited
+    /// -- and it is not a measurement of the prompt: it says the request is
+    /// out and the model has said nothing yet, which for a server serving one
+    /// slot is what reading the prompt looks like.
+    Reading,
     /// The session's own turn is over, but work it started is still going:
     /// a backgrounded subagent, or a command left running.
     ///