build: add receiver-only configuration and a Pi install script

The receiver does not need the sender's PipeWire/xdg-desktop-portal
capture stack, which small ARM boards neither have nor want. Add a
-Dsender=false meson option that skips the capture backend and the
sender pipeline (SC_HAS_SENDER guards in main.cpp/pipelines.cpp);
receiver-only builds refuse --send with a clear message while
--receive and --discover work unchanged.

scripts/install-receiver.sh targets such boards (e.g. Raspberry Pi
Zero 2 W): installs build and runtime dependencies via apt, checks the
compiler for C++20 <format> support before the long build (GCC 13+,
i.e. Raspberry Pi OS Trixie), builds SDL3 from source when the distro
does not package it, compiles a receiver-only binary, runs the test
suite, installs to /usr/local/bin, and enables avahi-daemon.

Also refresh the stale README (phases, dependencies, current
roadmap, Pi receiver section).

Validated in both configurations: meson test 5/5 each; the
receiver-only build has no PipeWire/portal references, refuses --send
cleanly, and --discover works headlessly.

README.md

@@ -4,21 +4,21 @@ A native Linux peer-to-peer screencast application.
 
 - **Send** your desktop or a window to another Linux machine.
 - **Receive** a stream and render it in a window.
-- Built with **C++20**, **Meson**, **PipeWire**, **FFmpeg**, **RTP/UDP**, and
-  **SDL**.
+- Discover receivers on the LAN via **mDNS/Avahi**; negotiate sessions with
+  **JSON signaling**; stream **H.264 over RTP/UDP**.
 
-> This project is in early development. The H.264 codec path is implemented;
-> capture, transport, and rendering are still in progress.
+Built with **C++20**, **Meson**, **PipeWire**, **FFmpeg**, and **SDL3**.
 
 ## Quick start
 
 Requirements:
 
-- C++20 compiler (GCC 12+, Clang 16+)
-- Meson >= 0.63
-- Ninja
+- C++20 compiler with `<format>` (GCC 13+, Clang 18+)
+- Meson >= 0.63, Ninja
 - FFmpeg development packages (`libavcodec`, `libavutil`, `libswscale`)
-- (later phases) PipeWire dev, SDL2/3 dev
+- SDL3 development package (`sdl3`)
+- nlohmann JSON (`nlohmann_json`) and Avahi client (`avahi-client`)
+- For the sender only: PipeWire dev (`libpipewire-0.3`) and libportal
 
 Build and run tests:
 
@@ -28,12 +28,35 @@ meson compile -C build
 meson test -C build --print-errorlogs
 ```
 
-Run with verbose test output:
+Stream between two machines:
 
 ```sh
-meson test -C build -v
+screencast --receive        # machine A: announces itself, opens a window
+screencast --send          # machine B: discovers A, negotiates, streams
 ```
 
+See `docs/RUNBOOK.md` for all modes, flags, and validation procedures.
+
+## Receiver on a small ARM board (e.g. Raspberry Pi Zero 2 W)
+
+The receiver does not need the sender's PipeWire/portal capture stack. On
+the board, run:
+
+```sh
+sudo ./scripts/install-receiver.sh
+screencast --receive
+```
+
+The script installs the dependencies, builds a receiver-only binary
+(`-Dsender=false`), runs the test suite, and installs to `/usr/local/bin`
+(override with `SC_RECEIVER_INSTALL_DIR`). It needs Raspberry Pi OS Trixie
+or newer (GCC 13+ for C++20 `<format>`), builds SDL3 from source when the
+distribution does not package it, and enables `avahi-daemon`.
+
+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.
+
 ## Architecture
 
 See `docs/ARCHITECTURE.md` for module boundaries and design rules.
@@ -42,8 +65,8 @@ See `docs/ARCHITECTURE.md` for module boundaries and design rules.
 
 Development is split into phases in `docs/PHASES.md`.
 
-Current phase: **Phase 3 — PipeWire screen capture**.
+Current phase: **Phase 7 — Resilience and polish**.
 
 ## License
 
-MIT — see `LICENSE` (to be added).
+MIT — see `LICENSE` (to be added).