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

config.example.yaml

@@ -26,7 +26,11 @@ player:
                                # interval (staggered) so latency can't slowly
                                # drift; 0 = off. 600 keeps drift to a few sec.
   restart_backoff_seconds: 3
-  extra_args: []
+  extra_args: []               # appended verbatim to every stream's mpv.
+                               # ["--panscan=1.0"] crops each feed to fill its
+                               # tile instead of letterboxing it — useful when
+                               # the grid's cells aren't 16:9, at the cost of the
+                               # top/bottom of the field of view.
 
 # Camera catalog. Normally populated by `rtsp-streamer discover`; shown here
 # by hand for illustration. Each camera can carry multiple stream qualities
@@ -94,10 +98,17 @@ clock:
   format: "15:04:05"           # Go time layout (24-hour w/ seconds). Others:
                                #   "3:04:05 PM"  ·  "Mon Jan 2  15:04"
   corner: bottom-right         # bottom-right | bottom-left | top-right | top-left
+                               #   | bottom-center | top-center
+                               # the *-center positions center horizontally and
+                               # ignore margin on that axis
   font_size: 44                # glyph height in px
   width: 300                   # overlay window size in px
   height: 72
   margin: 24                   # gap from the screen edges in px
+  background_opacity: 0.45     # alpha of the backing box behind the text.
+                               # MUST be > 0: on a fully transparent canvas the
+                               # glyphs inherit alpha 0 and nothing is drawn at
+                               # all. 0/absent is clamped to this default.
 
 # Re-sync the active layout from its linked UniFi Protect live view every N
 # seconds (0 = off). Layouts created by `views import` carry a `protect_view:`