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

README.md

@@ -104,6 +104,21 @@ on a peak of 32767.
 - A bar-length linter in the score parser catches more composition bugs
   than any amount of staring at note lists.
 
+## OPL3 rendering (the authentic one)
+
+For real Sound Blaster flavour — FM synthesis, no soundfont, like the
+actual YMF262 chip:
+
+```bash
+./tools/opl/build.sh      # one-time: fetches + builds libADLMIDI
+./tools/render_opl.sh     # renders all midi/*.mid through OPL3
+OPL_MODEL=4 ./tools/render_opl.sh midi/03_late_equaliser.mid
+```
+
+Volume models mimic real game sound drivers: `2` NativeOPL3 (id-style),
+`3` DMX (Doom, default), `4` APOGEE (Duke Nukem 3D), `8` AIL, `10` HMI.
+Output is normalised (OPL runs quiet). Needs `cmake` + `g++` to build.
+
 ## Licence & soundfonts
 
 MIT — see [LICENSE](LICENSE). Rendering needs a General MIDI soundfont,