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_test.go

@@ -0,0 +1,95 @@
+package evidence
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestPollLoopEmitsOnTitleChangeWithinSameWindow is the regression for the bug
+// where the X11 sensor only re-read the window on _NET_ACTIVE_WINDOW changes and
+// so missed a browser tab switch — same window class, new title — crediting all
+// the time to the stale title. The poll loop must emit when only the title
+// changes.
+func TestPollLoopEmitsOnTitleChangeWithinSameWindow(t *testing.T) {
+	// Same window (class) throughout; the title changes mid-stream, like a tab
+	// switch. The repeated identical reads must NOT re-emit; the title change must.
+	titles := []string{"Keel - Brave", "Keel - Brave", "Consume - Brave", "Consume - Brave"}
+	var i int
+	read := func() WindowSnapshot {
+		idx := i
+		if idx >= len(titles) {
+			idx = len(titles) - 1
+		}
+		i++
+		return WindowSnapshot{Class: "Brave-browser", Title: titles[idx], Health: EvidenceHealth{Available: true}}
+	}
+
+	var mu sync.Mutex
+	var got []WindowSnapshot
+	onChange := func(s WindowSnapshot) {
+		mu.Lock()
+		got = append(got, s)
+		mu.Unlock()
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() { pollLoop(ctx, time.Millisecond, read, onChange); close(done) }()
+
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		mu.Lock()
+		n := len(got)
+		mu.Unlock()
+		if n >= 2 {
+			break
+		}
+		if time.Now().After(deadline) {
+			cancel()
+			<-done
+			t.Fatalf("expected >=2 emits (one per distinct title), got %d", n)
+		}
+		time.Sleep(time.Millisecond)
+	}
+	cancel()
+	<-done
+
+	mu.Lock()
+	defer mu.Unlock()
+	if got[0].Title != "Keel - Brave" {
+		t.Fatalf("first emit title = %q, want %q", got[0].Title, "Keel - Brave")
+	}
+	if got[1].Title != "Consume - Brave" {
+		t.Fatalf("second emit title = %q, want %q (a title-only change must emit)", got[1].Title, "Consume - Brave")
+	}
+}
+
+// TestPollLoopDedupesIdenticalObservations confirms an unchanged observation is
+// not re-emitted, so redundant samples never churn the consumer.
+func TestPollLoopDedupesIdenticalObservations(t *testing.T) {
+	read := func() WindowSnapshot {
+		return WindowSnapshot{Class: "code", Title: "main.go", Health: EvidenceHealth{Available: true}}
+	}
+	var mu sync.Mutex
+	var n int
+	onChange := func(WindowSnapshot) {
+		mu.Lock()
+		n++
+		mu.Unlock()
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	done := make(chan struct{})
+	go func() { pollLoop(ctx, time.Millisecond, read, onChange); close(done) }()
+	time.Sleep(50 * time.Millisecond)
+	cancel()
+	<-done
+
+	mu.Lock()
+	defer mu.Unlock()
+	if n != 1 {
+		t.Fatalf("identical observations must emit once, got %d emits", n)
+	}
+}