Round-off: README render routes, tune scripts create midi/ if missing

Fresh-clone test passes end-to-end: clone -> build_all.sh -> all four
tunes generate, verify, render and pass audio checks with no prior state.

README.md

@@ -31,8 +31,10 @@ python3 tests/smoke_test.py   # library self-test
 python3 tools/make_tune.py tunes/specs/04_tea_break_at_the_terminal.json --render
 ```
 
-Rendering needs `timidity` + a GM soundfont (`FluidR3_GM.sf2`). Note:
-Debian's timidity build has no `-O2` mode — use `-Ow -s 44`.
+Rendering needs `timidity` + a GM soundfont (`FluidR3_GM.sf2`, MIT —
+`apt install fluid-soundfont-gm`), or skip soundfonts entirely and render
+through OPL3 emulation — see [OPL3 rendering](#opl3-rendering-the-authentic-one).
+Note: Debian's timidity build has no `-O2` mode — use `-Ow -s 44`.
 
 **AI agents**: see [AGENTS.md](AGENTS.md) — the JSON spec format is the
 agent-friendly front door; no Python required to make a tune.