Add Windows polling active-window sensor

internal/evidence/source_other.go

@@ -1,4 +1,4 @@
-//go:build !linux
+//go:build !linux && !windows
 
 package evidence
 

internal/evidence/windows.go

@@ -0,0 +1,51 @@
+//go:build windows
+
+package evidence
+
+import (
+	"context"
+	"time"
+
+	"antidrift/internal/winapi"
+)
+
+// pollInterval is how often the Windows sensor samples the foreground window.
+// ~1s latency on a window switch is immaterial for a focus tracker, and polling
+// avoids the message-loop/callback machinery a SetWinEventHook source needs.
+const pollInterval = 750 * time.Millisecond
+
+// NewSource returns the Windows active-window sensor (polling).
+func NewSource() Source { return windowsSource{} }
+
+type windowsSource struct{}
+
+// Watch emits the current window immediately, then samples every pollInterval,
+// emitting only when the foreground window or its title changes. A read with no
+// foreground window yields an Unavailable snapshot (once, until it recovers). It
+// runs until ctx is cancelled. It never panics the daemon.
+func (windowsSource) Watch(ctx context.Context, onChange func(WindowSnapshot)) {
+	var tr foregroundTracker
+	poll := func() {
+		hwnd, title, class, ok := winapi.ForegroundWindow()
+		if !tr.changed(ok, hwnd, title) {
+			return
+		}
+		if !ok {
+			onChange(WindowSnapshot{Health: EvidenceHealth{Available: false, Reason: "no foreground window"}})
+			return
+		}
+		onChange(WindowSnapshot{Title: title, Class: class, Health: EvidenceHealth{Available: true}})
+	}
+
+	poll() // immediate current window
+	ticker := time.NewTicker(pollInterval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			poll()
+		}
+	}
+}