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.

meson_options.txt

@@ -3,3 +3,6 @@ option('tests', type : 'boolean', value : true,
 
 option('gui', type : 'boolean', value : false,
     description : 'Build the optional GUI target (deferred to a later phase)')
+
+option('sender', type : 'boolean', value : true,
+    description : 'Build the sender; requires the PipeWire and xdg-desktop-portal development packages. Disable for receiver-only targets such as small ARM boards.')