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/opl/build.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# build.sh — fetch and build libADLMIDI, then compile render_opl.
+# Idempotent: skips anything already present. Run once per machine.
+set -euo pipefail
+cd "$(dirname "$0")"
+LIBDIR="${OPL_LIBDIR:-/opt/libADLMIDI}"
+
+if [ ! -f "$LIBDIR/build/libADLMIDI.a" ]; then
+    echo "cloning libADLMIDI..."
+    git clone --depth 1 -q https://github.com/Wohlstand/libADLMIDI.git "$LIBDIR"
+    echo "building (a few minutes)..."
+    cmake -B "$LIBDIR/build" -S "$LIBDIR" -DCMAKE_BUILD_TYPE=Release >/dev/null
+    cmake --build "$LIBDIR/build" -j"$(nproc)" --target ADLMIDI_static >/dev/null
+fi
+
+g++ -O2 -I "$LIBDIR/include" render_opl.cpp "$LIBDIR/build/libADLMIDI.a" \
+    -o render_opl -lpthread
+echo "built: $(pwd)/render_opl"