sensord: report edges, not every sample

Light drifts continuously; monitor-sensor emitted ~8 lines/sec and each was
a socket round trip re-asserting the same Changing(false). The keepalive is
what keeps silence meaningful, so only changes need reporting.

src/bin/souveraine-sensord.rs

@@ -283,12 +283,25 @@ fn main() {
                     continue;
                 }
                 let Some(value) = (s.parse)(&line, &mut state) else { continue };
-                {
+                // Only report when the value the machine sees actually
+                // changes. monitor-sensor emits a line per lux sample and
+                // ambient light drifts continuously — measured on device at
+                // ~8 lines/second, every one of them a socket round trip
+                // carrying `Changing(false)` again. The keepalive already
+                // guarantees sessiond hears from every source inside
+                // SOURCE_DOWN_AFTER, so silence between real changes is safe
+                // and is exactly what §10 arranged for.
+                let unchanged = {
                     let mut guard = match last.lock() {
                         Ok(g) => g,
                         Err(p) => p.into_inner(),
                     };
+                    let same = guard.get(s.name) == Some(&value);
                     guard.insert(s.name, value.clone());
+                    same
+                };
+                if unchanged {
+                    break;
                 }
                 // Report both edges. "Far" matters as much as "near": it is
                 // what re-enables a wake, and it is what proves the sensor is