cross last-seen over the wire as seconds-ago per source

TASK-08(f): EvidenceSeen held the stamps internally and nothing outside
the daemon could read them. to_ipc_json now carries
evidence_last_seen_secs_ago; null stays distinct from stale, because
never-reported and long-ago are different claims.

src/sessiond/device_state.rs

@@ -3315,6 +3315,15 @@ impl DeviceStateMachine {
                 "light": self.is_fresh(self.evidence_seen.light),
                 "touch": self.is_fresh(self.evidence_seen.touch),
             },
+            // Seconds since each source last reported; null means never.
+            // `fresh` is a threshold answer, this is the raw number — a page
+            // rendering "last seen 4 s ago" needs the number, not the bool.
+            "evidence_last_seen_secs_ago": {
+                "proximity": Self::secs_since(self.evidence_seen.proximity),
+                "accel": Self::secs_since(self.evidence_seen.accel),
+                "light": Self::secs_since(self.evidence_seen.light),
+                "touch": Self::secs_since(self.evidence_seen.touch),
+            },
             // Health is the other axis: `fresh` says whether the reading may
             // be believed, `health` says whether the source is there at all.
             "sensor_health": self.source_health.as_json(),
@@ -3327,6 +3336,10 @@ impl DeviceStateMachine {
             Instant::now().saturating_duration_since(t) <= self.policy.evidence_ttl
         })
     }
+
+    fn secs_since(seen: Option<Instant>) -> Option<u64> {
+        seen.map(|t| Instant::now().saturating_duration_since(t).as_secs())
+    }
 }
 
 impl fmt::Display for DeviceState {
@@ -4966,6 +4979,34 @@ mod tests {
         assert_eq!(json["evidence_fresh"]["proximity"], serde_json::json!(true));
     }
 
+    #[test]
+    fn last_seen_crosses_the_wire_as_seconds_ago() {
+        // TASK-08(f): the machine held last-seen internally and nothing
+        // outside could read it. null is "never reported", a different claim
+        // from "long ago".
+        let mut sm = DeviceStateMachine::new();
+        let json = sm.to_ipc_json();
+        assert_eq!(
+            json["evidence_last_seen_secs_ago"]["proximity"],
+            serde_json::Value::Null
+        );
+
+        sm.mark_evidence_seen_at(
+            SensorSource::Proximity,
+            Instant::now() - Duration::from_secs(30),
+        );
+        let json = sm.to_ipc_json();
+        let secs = json["evidence_last_seen_secs_ago"]["proximity"]
+            .as_u64()
+            .expect("a stamp 30 s old crosses as a number");
+        assert!((30..=31).contains(&secs), "got {secs}");
+        assert_eq!(
+            json["evidence_last_seen_secs_ago"]["light"],
+            serde_json::Value::Null,
+            "a source that never reported stays null"
+        );
+    }
+
     #[test]
     fn a_repeated_reading_is_not_a_trail_entry() {
         // Reporters heartbeat every 30s so silence is meaningful. If each