Add on-screen clock overlay (outlined text, configurable corner/timezone)

A small always-on-top mpv window renders the local time in a screen
corner. Implementation notes:

- Transparent lavfi canvas (color=...@0.0, --alpha=yes) so the camera
  video shows through; no new dependencies. If the compositor can't do
  alpha it degrades to a dark backing, still legible.
- Time drawn as an ASS osd-overlay pushed over mpv IPC once a second:
  white fill + black outline (\bord), so it reads on both bright (day)
  and dark (night) scenes without sampling the picture. Formatting the
  text in Go avoids any filtergraph escaping.
- Time computed with time.LoadLocation against a configured IANA zone
  (default "Local"), so it's correct regardless of the host clock's zone
  and handles DST. A bad zone name fails at startup.
- Managed on the daemon's own context (survives layout switches); the
  daemon keeps it positioned and raised above camera tiles. ensureClock
  is a no-op when the clock config + resolution are unchanged, so a
  reload never disturbs it.

Config: new `clock` section (enabled, timezone, format, corner,
font_size, width, height, margin) with defaults and corner validation.
Documented in README and config.example (shipped enabled, America/Denver,
24-hour w/ seconds). Tests cover corner geometry.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/daemon/daemon.go

@@ -41,6 +41,10 @@ type Daemon struct {
 	cancelLo context.CancelFunc // cancels the current layout's supervisors
 	wg       sync.WaitGroup
 	pcl      *protect.Client // cached Protect session for view sync
+
+	clockSig    string             // config signature of the running clock
+	clockCancel context.CancelFunc // stops the clock supervisor
+	clockWG     sync.WaitGroup
 }
 
 // New constructs a daemon bound to a config path. logs, when non-nil, is the
@@ -74,6 +78,7 @@ func (d *Daemon) Run(ctx context.Context) error {
 	<-ctx.Done()
 	d.log.Info("shutting down")
 	d.stopLayout()
+	d.stopClock()
 	return nil
 }
 
@@ -98,6 +103,10 @@ func (d *Daemon) reload(ctx context.Context) error {
 	d.mu.Lock()
 	d.cfg = cfg
 	d.mu.Unlock()
+	// The clock overlay is independent of the layout, so manage it here on the
+	// daemon's own context — it survives layout switches instead of being torn
+	// down and rebuilt with each wall.
+	d.ensureClock(ctx, cfg)
 	name := cfg.ActiveLayout
 	if name == "" {
 		d.log.Warn("no active_layout set; nothing to display")
@@ -107,6 +116,81 @@ func (d *Daemon) reload(ctx context.Context) error {
 	return d.applyLayout(ctx, name)
 }
 
+// ensureClock starts, stops, or restarts the clock overlay to match cfg. It is
+// a no-op when the clock config (and output resolution) is unchanged, so a
+// routine reload never disturbs a running clock.
+func (d *Daemon) ensureClock(ctx context.Context, cfg *config.Config) {
+	w, h := d.resolution(ctx, cfg)
+	sig := ""
+	if cfg.Clock.Enabled {
+		sig = fmt.Sprintf("%dx%d|%+v", w, h, cfg.Clock)
+	}
+	if sig == d.clockSig {
+		return
+	}
+	d.stopClock()
+	d.clockSig = sig
+	if !cfg.Clock.Enabled {
+		return
+	}
+	clock, err := player.NewClock(cfg.Clock, d.runDir, d.log)
+	if err != nil {
+		d.log.Warn("clock disabled", "err", err)
+		d.clockSig = ""
+		return
+	}
+	rect := clockRect(w, h, cfg.Clock)
+	clkCtx, cancel := context.WithCancel(ctx)
+	d.clockCancel = cancel
+	d.clockWG.Add(2)
+	go func() { defer d.clockWG.Done(); clock.Supervise(clkCtx) }()
+	go func() { defer d.clockWG.Done(); d.placeClock(clkCtx, clock, rect) }()
+	d.log.Info("clock overlay enabled", "corner", cfg.Clock.Corner, "rect", rect)
+}
+
+// stopClock tears down a running clock overlay, if any.
+func (d *Daemon) stopClock() {
+	if d.clockCancel == nil {
+		return
+	}
+	d.clockCancel()
+	d.clockCancel = nil
+	d.clockWG.Wait()
+}
+
+// placeClock keeps the clock window in its corner and on top. It re-asserts on
+// a slow tick because a layout switch remaps camera windows that would
+// otherwise stack over the overlay.
+func (d *Daemon) placeClock(ctx context.Context, clock *player.Clock, rect compositor.Rect) {
+	for {
+		if !sleepCtx(ctx, 2*time.Second) {
+			return
+		}
+		pid := clock.PID()
+		if pid == 0 {
+			continue
+		}
+		if err := d.comp.PlaceAndRaise(ctx, pid, rect); err != nil {
+			d.log.Debug("clock place failed", "err", err)
+		}
+	}
+}
+
+// clockRect computes the overlay rectangle for the configured corner.
+func clockRect(w, h int, cl config.Clock) compositor.Rect {
+	m, cw, ch := cl.Margin, cl.Width, cl.Height
+	x, y := w-cw-m, h-ch-m // bottom-right default
+	switch cl.Corner {
+	case "bottom-left":
+		x, y = m, h-ch-m
+	case "top-left":
+		x, y = m, m
+	case "top-right":
+		x, y = w-cw-m, m
+	}
+	return compositor.Rect{X: x, Y: y, W: cw, H: ch}
+}
+
 // tileSpec is one resolved tile: which stream goes where. It doubles as the
 // unit of the wall signature — if the specs (plus resolution and player
 // settings) are unchanged, a reload need not touch the running streams.