OTA step 2: signing key, Update File builder, push client

- scripts/ota_keygen.sh: ECDSA P-256 key pair; the private key goes to
  ~/.config/roro9stack/ (0600), the public key to keys/ and
  src/platform/ota_public_key.h; .gitignore refuses *key.pem
- scripts/make_ota.py: wraps firmware.bin into a signed .ota (openssl)
- scripts/ota_push.py: sends it over TCP 3232, prints the device's answer
- scripts/flash.sh --ota <host>: build, sign, push

Checked: a generated .ota has the documented layout and its signature
verifies with openssl against the committed public key.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

scripts/ota_keygen.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# Creates the Firmware Update signing key pair, once (ADR 0003).
+# The private key stays in ~/.config/roro9stack/ and must never be committed; the public key is
+# written into the firmware source. Losing the private key means the next update goes over USB.
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+KEY="${RORO_OTA_KEY:-$HOME/.config/roro9stack/ota-key.pem}"
+PUB_PEM="$ROOT/keys/ota-public.pem"
+PUB_H="$ROOT/src/platform/ota_public_key.h"
+
+if [ -e "$KEY" ]; then
+    echo "A signing key already exists at $KEY; not overwriting it." >&2
+    exit 1
+fi
+mkdir -p "$(dirname "$KEY")" "$ROOT/keys"
+( umask 077; openssl ecparam -name prime256v1 -genkey -noout -out "$KEY" )
+openssl ec -in "$KEY" -pubout -out "$PUB_PEM" 2>/dev/null
+
+{
+    echo "#pragma once"
+    echo ""
+    echo "// Public key that Firmware Updates must be signed for (ECDSA P-256). Generated by"
+    echo "// scripts/ota_keygen.sh; the private key is not in this repository (ADR 0003)."
+    echo "namespace roro {"
+    echo "inline constexpr char kOtaPublicKeyPem[] ="
+    sed 's/^/    "/; s/$/\\n"/' "$PUB_PEM"
+    echo "    ;"
+    echo "}  // namespace roro"
+} > "$PUB_H"
+echo "Private key: $KEY (keep it safe)"
+echo "Public key:  $PUB_PEM and $PUB_H (commit these)"