Rebuild as a code-first MIDI composition toolkit

Replaces the April wreckage (fifteen 70-byte 'working' MIDIs, archived)
with a proper pipeline:

- lib/midikit.py: stdlib-only SMF format-1 writer + reader/verifier
- lib/songkit.py: composition helpers (bar-grid score DSL with a 4/4
  bar linter, OPL arps, driving bass, vibrato, drum patterns)
- tunes/: three original tunes - DOS adventure theme (Dm/132),
  N64 ambient (G/72), chiptune racing (Em/152) - all with LOOP_START/
  LOOP_END markers for game engines
- tools/build_all.sh + tools/check_wav.py: generate, verify, render
  via timidity+FluidR3, fail on clipping/dead air
- tests/smoke_test.py: round-trip verified against mido

All three tunes render clean: no clipping, no dead air, durations
match, chromaticism audited against the chord chart.

tests/smoke_test.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+"""Smoke test: does midikit+songkit produce a valid, parseable, playable file?
+Run from repo root: python3 tests/smoke_test.py"""
+import os, sys
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "lib"))
+from midikit import MidiFile, Track, MPQN, note, note_name, read, verify
+from songkit import bar, melody, arp, bass_drive, drums, vibrato, humanize, parse_score
+
+assert note("A4") == 69 and note("C#5") == 73 and note("Bb3") == 58
+assert note_name(69) == "A4" and note_name(60) == "C4"
+
+mf = MidiFile(tpq=480)
+mf.time_sig = (4, 2)
+mf.key_sig = (-1, True)  # D minor
+lead = Track("Lead", channel=0)
+lead.program(0, "square lead")
+melody(lead, 0, "D4 A4-8 D5-8 F5-8 E5-4 D5-2")
+vibrato(lead, bar(0), bar(4), channel=0, tempo_bpm=120)
+bass = Track("Bass", channel=1)
+bass.program(0, "synth bass 1")
+bass_drive(bass, 0, [("D", "m"), ("Bb", "")])
+acomp = Track("Arp", channel=2)
+acomp.program(0, "electric piano 1")
+arp(acomp, 0, [("D", "m"), ("Bb", "")])
+d = Track("Drums", channel=9)
+drums(d, 0, 2, "rock")
+mf.add_track(lead); mf.add_track(bass); mf.add_track(acomp); mf.add_track(d)
+mf.tempos = [MPQN.from_bpm(0, 120)]
+path = "/tmp/midikit_smoke.mid"
+mf.write(path)
+
+d = verify(path, min_sec=1, max_sec=30)
+print("OK:", path, d["duration_sec"], "sec,",
+      sum(len(t["notes"]) for t in d["tracks"]), "notes,",
+      len(d["tracks"]), "tracks")
+
+# cross-check with mido if present (third-party parser as second opinion)
+try:
+    import mido
+    m = mido.MidiFile(path)
+    print("mido agrees:", m.length, "sec,", len(m.tracks), "tracks")
+    assert abs(m.length - d["duration_sec"]) < 0.5
+except ImportError:
+    print("(mido not installed; skipping cross-check)")