fix(evidence): X11 sensor polls so tab/title changes are tracked

The X11 source only re-read the active window on _NET_ACTIVE_WINDOW
changes, so switching a browser tab — which changes the window title
but not the active window — was invisible. All the time on the new tab
was credited to the stale title (e.g. reading a Consume article showed
up as time on the Keel tab).

Mirror the Windows sensor: poll the active window every 750ms and emit
only when the window or its title changes. A shared, display-free
pollLoop carries the change-dedup and is unit-tested for the exact
regression (a title-only change must emit).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/evidence/poll.go

@@ -0,0 +1,44 @@
+package evidence
+
+import (
+	"context"
+	"time"
+)
+
+// pollLoop samples the active window via read and forwards snapshots through
+// onChange, but only when the observation changes — a different window OR a
+// different title. The title case is the one that matters: switching a browser
+// tab changes the window title without changing the active window, so a sensor
+// that re-reads only on active-window events never sees it and credits the stale
+// title. Polling re-reads on a fixed cadence, so a tab switch is caught within
+// one interval.
+//
+// It emits once immediately, then samples every interval, until ctx is
+// cancelled. WindowSnapshot is comparable, so the change check is a plain
+// equality. Factoring the loop here keeps the change-dedup identical across
+// sensors and lets it be tested without a display. (The Windows sensor predates
+// this and keeps its own equivalent loop.)
+func pollLoop(ctx context.Context, interval time.Duration, read func() WindowSnapshot, onChange func(WindowSnapshot)) {
+	var last WindowSnapshot
+	var haveLast bool
+	emit := func() {
+		s := read()
+		if haveLast && s == last {
+			return
+		}
+		last, haveLast = s, true
+		onChange(s)
+	}
+
+	emit() // immediate current window
+	t := time.NewTicker(interval)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			emit()
+		}
+	}
+}