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/run_gzip_test.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+#
+# Test gzip compression workflow:
+# 1. Create archive from first state file
+# 2. Compress with gzip
+# 3. Append remaining state files to the compressed archive
+# 4. Decompress and show info
+#
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_DIR="$(dirname "$(dirname "$SCRIPT_DIR")")"
+BINARY="$PROJECT_DIR/target/debug/json-archive"
+DATA_DIR="$SCRIPT_DIR/data"
+OUT_DIR="$SCRIPT_DIR/out/gzip"
+
+echo "=== Gzip Compression Test ==="
+
+# Setup
+rm -rf "$OUT_DIR"
+mkdir -p "$OUT_DIR"
+
+# Create initial archive from first state file
+echo "Creating archive from state_1.json..."
+"$BINARY" "$DATA_DIR/state_1.json" -o "$OUT_DIR/test.json.archive"
+
+# Compress with gzip
+echo "Compressing with gzip..."
+gzip "$OUT_DIR/test.json.archive"
+ls -la "$OUT_DIR/"
+
+# Append remaining files to compressed archive
+for i in $(seq 2 9); do
+    echo "Appending state_$i.json to compressed archive..."
+    "$BINARY" "$OUT_DIR/test.json.archive.gz" "$DATA_DIR/state_$i.json"
+done
+
+# Show info on the result
+echo ""
+echo "Final archive info:"
+"$BINARY" info "$OUT_DIR/test.json.archive.gz"
+
+# Decompress for manual inspection
+echo ""
+echo "Decompressing for comparison..."
+gunzip -k "$OUT_DIR/test.json.archive.gz" 2>/dev/null || gunzip -c "$OUT_DIR/test.json.archive.gz" > "$OUT_DIR/test.json.archive"
+
+echo ""
+echo "Decompressed archive info:"
+"$BINARY" info "$OUT_DIR/test.json.archive"
+
+echo ""
+echo "Files in $OUT_DIR:"
+ls -la "$OUT_DIR/"