feat: transparently append to compressed archives

When appending to a compressed archive (gzip, brotli, zstd), the tool
now handles compression automatically. Since some compression formats don't
support appending to compressed files in place, we write a new
compressed file with all the data and atomically rename it to replace
the original (assuming there is enough space on that filesystem).

This means you can work with compressed archives the same way as
uncompressed ones. Point the tool at your .json.gz file and append
values. No manual decompression/recompression needed.

tests/compression-integration/generate_state.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+"""
+Generate a JSON state file with N items in each array.
+Output goes to stdout.
+
+Usage: ./generate_state.py <n>
+"""
+
+import json
+import sys
+
+def main():
+    if len(sys.argv) != 2:
+        print("Usage: generate_state.py <n>", file=sys.stderr)
+        sys.exit(1)
+
+    n = int(sys.argv[1])
+
+    state = {
+        "colors": [f"color_{i}" for i in range(1, n + 1)],
+        "numbers": [f"number_{i}" for i in range(1, n + 1)],
+        "animals": [f"animal_{i}" for i in range(1, n + 1)],
+    }
+
+    print(json.dumps(state))
+
+if __name__ == "__main__":
+    main()