Fix single-file backward compatibility with original UC2 Pro

Root cause: the original UC2 Pro expects csize=0 in the cdir COMPRESS
record (it ignores the field entirely).  UC2 v3 was writing the actual
compressed size, which confused the original's archive reader.

Additional changes:
- Use default Huffman tree for all blocks (ensures tree encoding compat)
- Write method=compression_level in cdir COMPRESS (was hardcoded to 1)
- Add tests/scripts/bitdump.py for bit-level bitstream analysis

Single-file UC2 v3 archives are now fully readable by the original UC2
Pro (listing and extraction verified in DOSBox-X).  Multi-file archives
still hang — the cdir bitstream decodes correctly in our Python analyzer
but fails in the original's ASM decompressor kernel.  Investigation
continues; the bitdump.py tool enables targeted comparison.

tests/scripts/roundtrip_dosbox.sh

@@ -2,13 +2,10 @@
 # Cross-tool round-trip test: original UC2 Pro -> UC2 v3 via DOSBox-X
 #
 # Tests that archives created by the original 1992 UC2 Pro can be correctly
-# extracted by UC2 v3 (Direction 2).
-#
-# Direction 1 (UC2 v3 -> original) is not yet implemented — the original
-# UC2 Pro hangs reading UC2 v3 archives.  Root cause is under investigation:
-# tree generation and data encoding match the original, but some bitstream-
-# level difference remains (likely in the ASM decompressor's expectations
-# around tree serialization or block structure).
+# extracted by UC2 v3 (Direction 2).  Single-file UC2 v3 archives are also
+# readable by the original, but multi-file archives still hang — the exact
+# cause is under investigation (treegen + cdir encoding match the original
+# for single-file cdirs but diverge for multi-file cdirs).
 #
 # Usage: roundtrip_dosbox.sh <uc2-cli> <uc2pro.exe> <corpus-dir>
 
@@ -52,12 +49,11 @@ uc2pro UC2DIST
 exit
 DOSBOXCFG
 
-timeout 180 flatpak run com.dosbox_x.DOSBox-X \
+timeout 300 flatpak run com.dosbox_x.DOSBox-X \
     -conf "$WORK/dosbox.conf" -nopromptfolder 2>/dev/null || true
 
 if [ ! -f "$WORK/UC2DIST/UC.EXE" ]; then
     echo "FAIL: UC2 Pro SFX extraction did not produce UC.EXE"
-    ls -la "$WORK/UC2DIST/" 2>/dev/null || echo "UC2DIST directory missing"
     exit 1
 fi
 echo "  UC.EXE extracted ($(wc -c < "$WORK/UC2DIST/UC.EXE") bytes)"
@@ -86,25 +82,25 @@ timeout 300 flatpak run com.dosbox_x.DOSBox-X \
     -conf "$WORK/dosbox.conf" -nopromptfolder 2>/dev/null || true
 
 if [ ! -f "$WORK/MARKER.TXT" ]; then
-    echo "FAIL: DOSBox session did not complete (archive creation may have timed out)"
+    echo "FAIL: DOSBox session did not complete"
     exit 1
 fi
 
-# --- Verify Direction 2 (original -> UC2 v3) ---
-echo "--- Verifying Direction 2 ---"
-FAIL=0
 DOS_ARCHIVE=""
 for candidate in "$WORK/out/DOSTEST.UC2" "$WORK/out/dostest.uc2"; do
     [ -f "$candidate" ] && DOS_ARCHIVE="$candidate" && break
 done
 if [ -z "$DOS_ARCHIVE" ]; then
     echo "FAIL: UC2 Pro did not create DOSTEST.UC2"
-    ls -la "$WORK/out/" 2>/dev/null
     exit 1
 fi
+echo "  Archive created: $(wc -c < "$DOS_ARCHIVE") bytes"
 
+# --- Extract with UC2 v3 and verify ---
+echo "=== Extracting with UC2 v3 ==="
 "$UC2_CLI" -d "$WORK/output" "$DOS_ARCHIVE"
 
+FAIL=0
 for f in "${FILES[@]}"; do
     upper=$(echo "$f" | tr '[:lower:]' '[:upper:]')
     extracted=""
@@ -112,7 +108,7 @@ for f in "${FILES[@]}"; do
         [ -f "$candidate" ] && extracted="$candidate" && break
     done
     if [ -z "$extracted" ]; then
-        echo "  FAIL: $f not extracted by UC2 v3"
+        echo "  FAIL: $f not extracted"
         FAIL=1
         continue
     fi