OPL3 rendering path: authentic Sound Blaster FM, no soundfont

- tools/opl/render_opl.cpp: libADLMIDI harness - renders MIDI through
  OPL3 emulation with game-driver volume models (DMX/Doom, Apogee/Duke,
  AIL, HMI), normalised output, WAV header patched to actual length
- tools/opl/build.sh: one-shot fetch+build of libADLMIDI
- tools/render_opl.sh: batch render + check_wav for all tunes
- README: OPL3 section

tools/render_opl.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# render_opl.sh — render tunes through OPL3 FM (authentic Sound Blaster
+# flavour, no soundfont). Usage:
+#   ./tools/render_opl.sh [tune.mid ...]     (default: all midi/*.mid)
+#   OPL_MODEL=4 ./tools/render_opl.sh ...    (4 = Apogee/Duke Nukem volumes)
+#
+# Volume models: 2 NativeOPL3 (id-style)  3 DMX (Doom, default)
+#                4 APOGEE (Duke Nukem 3D)  8 AIL  10 HMI
+set -euo pipefail
+cd "$(dirname "$0")/.."
+[ -x tools/opl/render_opl ] || ./tools/opl/build.sh
+mkdir -p build/opl
+MODEL="${OPL_MODEL:-3}"
+
+files=("$@")
+if [ ${#files[@]} -eq 0 ]; then files=(midi/*.mid); fi
+
+for f in "${files[@]}"; do
+    dur=$(python3 -c "
+import sys; sys.path.insert(0,'lib')
+from midikit import read
+print(read('$f')['duration_sec'] + 3)")
+    out="build/opl/$(basename "${f%.mid}").wav"
+    echo "== $f -> $out (model $MODEL)"
+    ./tools/opl/render_opl "$f" "$out" "$dur" "$MODEL"
+    python3 tools/check_wav.py "$out" "$dur"
+done