feat: render minimal flight OSD from supervisor snapshots (#3)

README.md

@@ -15,11 +15,17 @@ Current capabilities:
 - validates protocol version, frame length and CRC-16;
 - logs channel changes instead of every frame;
 - starts/stops `fpv-video` from the configured CH6 power/lifecycle control;
+- publishes immutable flight OSD snapshots — final RC channels, arm state
+  derived from the configured arm channel, requested CH6 WFB power and
+  decoded-video presence — to a tmpfs state file without ever blocking the
+  UART, WFB or video paths; unchanged state is republished as a heartbeat
+  and the file is removed on shutdown;
 - starts an independent kiosk display before video, tolerates temporary X11 downtime without restarting the supervisor, and stops deterministically on shutdown;
 - keeps the fullscreen fallback/menu canvas below `fpv-video`, showing `Brak połączenia WFB` only while video is absent;
-- maps a transparent, input-transparent OSD overlay above `fpv-video` as the reserved flight OSD rendering surface: it never presents pixels over the video or intercepts its input.
+- maps a transparent, input-transparent OSD overlay above `fpv-video` as the flight OSD rendering surface: it never intercepts video input, and it punches its output shape only around the text it draws;
+- renders a deliberately minimal Viture OSD — arm state and requested CH6 WFB power — on that overlay while decoded video is present, refreshing on a slower cadence than the video so the pilot stream keeps priority; a stale snapshot (dead UART) stops rendering instead of freezing telemetry over live video.
 
-Future MIPI/touch targets use their own templates rather than sharing one global layout. The flight OSD will render on the existing transparent layer above video, preserving the pilot stream's priority.
+Future MIPI/touch targets use their own templates rather than sharing one global layout. The flight OSD renders on the existing transparent layer above video, preserving the pilot stream's priority.
 
 ## Installation on Orange Pi
 
@@ -55,6 +61,20 @@ serial:
 
 `/dev/ttyS0` is the Orange Pi boot/debug UART and must not be used for the permanently connected GX12.
 
+The flight OSD is configured alongside:
+
+```yaml
+osd:
+  state_path: /run/groundstation/osd-state.bin
+  arm_index: 4        # CH5/AUX1, zero-based
+  armed_threshold: 0  # armed while the arm channel stays above this
+```
+
+`groundstation-display` reads the same state file; both sides must agree on
+the path (override the renderer's with `groundstation-display -osd-state
+<path>` if you change `osd.state_path`). The state file carries only flight
+telemetry and never any credentials or keys.
+
 ## Runtime dependencies
 
 - X11 session on `:0` owned by `orangepi`; the kiosk waits for X11 and reconnects after display-manager restarts;