CI: tests and builds on every push, a signed release on a tag (#5)

The workflow runs on the runner's host and builds in the project's Docker
image through scripts/ci.sh, as on a developer's machine. A tag v*, or a
run by hand for an older tag, builds that tag's sources, signs the Update
File with the key held in the repository's secrets, checks the signature
against the public key in the sources, and publishes a Gitea release with
the .ota, the factory image, the ELF and checksums.

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

scripts/ota_verify.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+"""Checks an Update File (.ota) the way the device does, on a PC: header, signature, image hash.
+
+Usage: scripts/ota_verify.py <file.ota> [public key, default keys/ota-public.pem]
+Exits 0 and prints the version if a device would accept the file.
+"""
+import hashlib
+import os
+import struct
+import subprocess
+import sys
+import tempfile
+
+HEADER_SIZE = 160
+SIGNED_BYTES = 80
+
+
+def main():
+    if len(sys.argv) < 2:
+        sys.exit(__doc__)
+    root = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+    key = sys.argv[2] if len(sys.argv) > 2 else os.path.join(root, "keys", "ota-public.pem")
+    data = open(sys.argv[1], "rb").read()
+    if len(data) < HEADER_SIZE or data[:8] != b"RORO-OTA":
+        sys.exit("not an update file")
+    fmt, header_size, image_size = struct.unpack("<HHI", data[8:16])
+    if fmt != 1 or header_size != HEADER_SIZE:
+        sys.exit("unsupported update format")
+    image = data[HEADER_SIZE:]
+    if len(image) != image_size:
+        sys.exit(f"the header announces {image_size} bytes of image, the file has {len(image)}")
+    if hashlib.sha256(image).digest() != data[16:48]:
+        sys.exit("image corrupted (hash mismatch)")
+    version = data[48:80].split(b"\0")[0].decode()
+    (sig_len,) = struct.unpack("<H", data[80:82])
+    with tempfile.NamedTemporaryFile() as signed, tempfile.NamedTemporaryFile() as sig:
+        signed.write(data[:SIGNED_BYTES])
+        signed.flush()
+        sig.write(data[82:82 + sig_len])
+        sig.flush()
+        ok = subprocess.run(["openssl", "dgst", "-sha256", "-verify", key, "-signature", sig.name, signed.name],
+                            capture_output=True).returncode == 0
+    if not ok:
+        sys.exit("bad signature (wrong key)")
+    print(f"{sys.argv[1]}: {version}, {image_size} bytes, signature good")
+
+
+if __name__ == "__main__":
+    main()