feat(render): fullscreen headless receiver with letterboxing

A headless receiver has no window manager, so a windowed window is
meaningless there — the screencast should simply fill the screen. The
renderer now goes borderless fullscreen automatically when SDL runs
the KMSDRM backend, and --fullscreen forces the same behavior in
desktop sessions. The video keeps its aspect ratio via
SDL_SetRenderLogicalPresentation(LETTERBOX): a 4:3 desktop on a 16:9
TV renders with black bars instead of a stretched picture, the cursor
is hidden, and the clear-before-draw keeps the bars black.

Verified live on a 3440x1440 monitor: the --fullscreen receiver window
covered the entire display while a 4:3 synthetic feed rendered with
pure-black pillarbox bars (Y=0/SAT=0) and a saturated-red center
(V=254). meson test 5/5 in both build configurations.

README.md

@@ -57,7 +57,9 @@ installs and enables a systemd service so the receiver starts at boot —
 
 Performance note: decoding is software H.264; on very small boards expect
 smooth playback for modest resolutions and reduced frame rates at high
-resolutions. Hardware decode is planned for Phase 7.
+resolutions. Hardware decode is planned for Phase 7. On a headless console
+the receiver runs fullscreen automatically with aspect-preserving
+letterboxing.
 
 ## Architecture