Fix invisible/flashing clock, map Protect's 8-camera preset, warn on gaps

Three separate faults made cameras "not load" and the clock misbehave.

Clock rendered as nothing, or flashed ~200ms/second. The time was drawn as
an ASS osd-overlay pushed over mpv IPC, but on mpv 0.35 + Mesa/V3D + sway an
OSD overlay is rendered only on the frame where its content *changes*. Every
layer reports success while this happens (mpv returns error:success,
vo-configured is true, and sway reports the window visible at the right
rect), so it looks like a stacking or font bug and is neither. Ruled out:
pushing at 20Hz (identical content is ignored, so it still only redrew when
the second flipped), osd-msg1, show-text, and --pause (mpv stops redrawing
entirely). Fonts were never the issue.

The time is now baked into every frame by a drawtext filter re-reading a
small file the ticker rewrites once a second, with two constraints that cost
real time to find and are pinned by tests:

- The canvas alpha must be > 0. A fully transparent canvas (black@0.0 with
  --alpha=yes) makes the glyphs inherit alpha 0 and the compositor draws
  nothing -- this was the original invisible clock. New clock
  background_opacity (default 0.45) is clamped in config *and* in args() so
  no code path can produce an invisible clock.
- Readahead must be off. drawtext stamps the time when a frame is
  *generated*, so buffering ahead makes the visible clock lag by the
  readahead and swallows text-file updates entirely.

Since the text now arrives through a file, the clock needs no IPC socket:
dropped --input-ipc-server, the ipcPath field, and the stale-socket removal.
assEscape goes with the ASS path.

`views import` produced layouts with holes. viewmap derived the grid from
the slot count alone and ignored Protect's `layout` field, so Protect's
asymmetric 8-camera preset (four 2x2 tiles plus a right column of four 1x1)
landed as 8 tiles in a 3x3 grid -- the bottom-right cell was simply empty
and rendered as a blank rectangle. That preset is now mapped exactly; other
counts keep the uniform GridForSlots fallback rather than guessing at
presets I have not observed. Import also warns when a mapping would leave
empty cells or references a camera missing from the config, so a silent hole
cannot reach the screen again.

Also:

- clock.corner gains bottom-center and top-center (centered horizontally,
  Margin still applies vertically).
- placeClock no longer re-issues `resize set` every tick. Re-asserting
  geometry on a correctly-sized window makes sway send a configure event,
  which makes mpv reallocate buffers and blank for a frame. New
  compositor.Raise re-asserts z-order only, which is all the 2s tick needs;
  geometry is re-placed only when it has actually drifted.
- README documents why the clock is drawn this way, the preset table and how
  to add another from `views dump`, and three troubleshooting entries for
  failure modes that all look like bugs: a blank tile whose mpv is running
  (a stale camera entry -- re-adopting a camera in Protect assigns a new id
  and often a slightly different name, and `discover` never prunes), cameras
  in `cameras:` not being on screen (only the active layout's tiles stream),
  and black bars inside tiles (non-16:9 grid cells; --panscan=1.0 crops to
  fill instead).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LYhTnkp7VzJ67THeicgfAQ

internal/config/config.go

@@ -59,7 +59,8 @@ type Clock struct {
 	// Examples: "3:04:05 PM", "Mon Jan 2  15:04".
 	Format string `yaml:"format,omitempty"`
 	// Corner places the overlay: bottom-right (default), bottom-left,
-	// top-right, top-left.
+	// top-right, top-left, bottom-center, top-center. The *-center positions
+	// center the overlay horizontally and ignore Margin on that axis.
 	Corner string `yaml:"corner,omitempty"`
 	// FontSize is the glyph height in pixels (default 44).
 	FontSize int `yaml:"font_size,omitempty"`
@@ -68,6 +69,15 @@ type Clock struct {
 	Height int `yaml:"height,omitempty"`
 	// Margin is the gap from the screen edges in pixels (default 24).
 	Margin int `yaml:"margin,omitempty"`
+	// BackgroundOpacity is the alpha of the overlay's backing box, 0.0
+	// (invisible) to 1.0 (solid black). Default 0.45.
+	//
+	// It must not be 0: the time is drawn into the canvas by a drawtext filter,
+	// and on a fully transparent canvas the glyphs inherit the canvas's zero
+	// alpha, so the compositor draws nothing and the clock silently disappears.
+	// A small non-zero value gives the text a dark backing that also keeps it
+	// legible over bright daytime scenes.
+	BackgroundOpacity float64 `yaml:"background_opacity,omitempty"`
 }
 
 // Controller holds UniFi Protect connection details.
@@ -355,6 +365,14 @@ func (c *Config) Defaults() {
 		if c.Clock.Margin == 0 {
 			c.Clock.Margin = 24
 		}
+		if c.Clock.BackgroundOpacity <= 0 {
+			// 0 renders an invisible clock (see BackgroundOpacity), so treat
+			// unset — and any nonsense value — as the default.
+			c.Clock.BackgroundOpacity = 0.45
+		}
+		if c.Clock.BackgroundOpacity > 1 {
+			c.Clock.BackgroundOpacity = 1
+		}
 	}
 }
 
@@ -407,9 +425,10 @@ func (c *Config) Validate() error {
 
 	if c.Clock.Enabled && c.Clock.Corner != "" {
 		switch c.Clock.Corner {
-		case "bottom-right", "bottom-left", "top-right", "top-left":
+		case "bottom-right", "bottom-left", "top-right", "top-left",
+			"bottom-center", "top-center":
 		default:
-			return fmt.Errorf("clock.corner %q must be one of bottom-right, bottom-left, top-right, top-left", c.Clock.Corner)
+			return fmt.Errorf("clock.corner %q must be one of bottom-right, bottom-left, top-right, top-left, bottom-center, top-center", c.Clock.Corner)
 		}
 	}
 	return nil