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/validate.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+#
+# Validate that compressed and decompressed archives produce the same results.
+# Run this after run_all.sh to smoke test the outputs.
+#
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_DIR="$(dirname "$(dirname "$SCRIPT_DIR")")"
+BINARY="$PROJECT_DIR/target/debug/json-archive"
+
+echo "=== Validation ==="
+
+errors=0
+
+for format in gzip brotli zstd; do
+    dir="$SCRIPT_DIR/out/$format"
+
+    if [ ! -d "$dir" ]; then
+        echo "SKIP: $format (no output directory)"
+        continue
+    fi
+
+    # Find the compressed and uncompressed files
+    compressed=$(find "$dir" -name "*.gz" -o -name "*.br" -o -name "*.zst" | head -1)
+    uncompressed="$dir/test.json.archive"
+
+    if [ ! -f "$compressed" ] || [ ! -f "$uncompressed" ]; then
+        echo "SKIP: $format (missing files)"
+        continue
+    fi
+
+    # Compare state output
+    state_compressed=$("$BINARY" state "$compressed")
+    state_uncompressed=$("$BINARY" state "$uncompressed")
+
+    if [ "$state_compressed" = "$state_uncompressed" ]; then
+        echo "OK: $format - state matches"
+    else
+        echo "FAIL: $format - state differs"
+        errors=$((errors + 1))
+    fi
+
+    # Compare observation count from info
+    count_compressed=$("$BINARY" info "$compressed" --output json | python3 -c "import sys,json; print(json.load(sys.stdin)['observation_count'])")
+    count_uncompressed=$("$BINARY" info "$uncompressed" --output json | python3 -c "import sys,json; print(json.load(sys.stdin)['observation_count'])")
+
+    if [ "$count_compressed" = "$count_uncompressed" ]; then
+        echo "OK: $format - observation count matches ($count_compressed)"
+    else
+        echo "FAIL: $format - observation count differs ($count_compressed vs $count_uncompressed)"
+        errors=$((errors + 1))
+    fi
+done
+
+echo ""
+if [ $errors -eq 0 ]; then
+    echo "All validations passed."
+else
+    echo "$errors validation(s) failed."
+    exit 1
+fi