fix(slice-1): F6 — drop dead Channel.created_at field

Adversarial review F6 (Low, confirmed): Channel.created_at: Instant was
drafted 'for the idle timeout' (spec §5.1) but the timer keys off
RtcSession.last_rtp_rx (and now last_rtp_rx after F4), never creation
time. The field was never read; the Instant import it pulled is dropped
with it.

Spec §5.1 already reconciled in 8680f2b (struct sketch + comment).
Spec §4.5 already specified 'no RTP' (60 s); the contradiction with
the old 5-min lineage is closed.

Refactor phase — full test suite + clippy green; no behavior change.

crates/rutster-call-model/src/lib.rs

@@ -47,7 +47,6 @@ mod tests {
     }
 }
 
-use std::time::Instant;
 use uuid::Uuid;
 
 /// Newtype wrapping a `Uuid` for the channel id.
@@ -127,17 +126,17 @@ pub enum Direction {
 /// - `media: Option<MediaLeg>` — second consumer.
 /// - `audiohooks: Vec<AudiohookHandle>` — escalation rung 2.
 /// - `tap: Option<TapHandle>` — step 2.
+///
+/// (F6 review patch: an earlier sketch carried `created_at: Instant` "for
+/// the idle timeout," but the timer keys off `RtcSession.last_rtp_rx`, not
+/// creation time. The field was dead; the work-pool would have populated
+/// it on every `new_inbound` and never read it. Dropped along with its
+/// `Instant` import.)
 #[derive(Debug)]
 pub struct Channel {
     pub id: ChannelId,
     pub state: ChannelState,
     pub direction: Direction,
-    /// For the 60 s idle timeout (spec §4.5). `Instant` is a monotonic
-    /// clock — choosing it over `SystemTime` means we're measuring
-    /// elapsed wall-time within this process, NOT a calendar time the
-    /// user could change mid-call. The monotonic clock is the right
-    /// tool for "has this peer been silent for 60 seconds?"
-    pub created_at: Instant,
 }
 
 impl Channel {
@@ -147,7 +146,6 @@ impl Channel {
             id: ChannelId::new(),
             state: ChannelState::New,
             direction: Direction::Inbound,
-            created_at: Instant::now(),
         }
     }
 }