docs: record the on-Pi low-RAM build as hardware-validated

.agents/MEMORY.md

@@ -50,6 +50,12 @@ loopback; current phase is Phase 7.
   service then browses but never resolves (timeout for every peer) until
   records expire (~75 min). `systemctl restart avahi-daemon` clears it.
   Recorded in RUNBOOK; always stop with SIGTERM.
+- **Low-RAM build validated on hardware**: the Zero 2 W receiver build
+  completes with the install script's temporary-swap + single-job handling
+  (the user confirmed; nlohmann/json's signaling TU was the OOM trigger
+  before the fix). Cross-compiling on the dev machine was considered and
+  dropped — the on-Pi build works and the toolchain/container effort was
+  not needed.
 
 - **Phase 5 (local UDP sender→receiver loopback) is implemented**:
   - `UdpRtpTransport` (src/network/udp_transport.cpp): raw POSIX sockets,