Harden for agent users: spec validation, key-sig accidentals, N64 chord qualities

- make_tune: validate_spec() reports every spec problem in one run
  (missing keys, unknown style, bad qualities, bpm<=0, out-of-range
  loop/fills, melody longer than chart, unsafe name) — no raw KeyErrors
- make_tune: key_sig_for handles accidentals in minor and major keys,
  any case (Bb minor, F# minor, bb major); real spellings kept (C# major)
- make_tune: n64 pad uses QUALITIES table (dim/aug/sus now correct)
- make_tune: WARNING on stderr for fields a style can't use (counter
  for non-DOS, fills_at for n64) instead of silent drops
- make_tune: name sanitised ([A-Za-z0-9][A-Za-z0-9._-]*), no traversal
- make_tune: render errors actionable (timidity presence check, stderr
  surfaced, sys.executable instead of bare python3)
- songkit: parse_score lints the FINAL bar too (was unchecked); this
  caught a real 5-beat bar in 01_service_station_2am COUNTER_A (fixed)
- build_all.sh: dependency + soundfont checks, removes stale midi/wav
  outputs with no current source, fails if nothing was produced
- smoke_test: runs every spec, checks loop marker tick positions,
  feeds malformed bars to the linter, covers validation + key sigs
- AGENTS.md: documents 0-based bars and exclusive end_bar, validation
  behaviour; SKILL.md added so the repo loads as an agent skill

lib/songkit.py

@@ -33,9 +33,9 @@ def parse_score(text, tpq=TPQ):
       rests: r4 / r8 / r2 / r16 / r1
 
     Bars may be separated with '|'. If any '|' is present, every bar
-    segment must total exactly 4 quarter notes — a free bar-length
-    linter that has saved me from more off-by-one-eighth bugs than
-    I care to admit.
+    segment — including the final one — must total exactly 4 quarter
+    notes — a free bar-length linter that has saved me from more
+    off-by-one-eighth bugs than I care to admit.
 
     Example: 'D4 A4-8 D5-8 F5-8 E5-4 D5-2'
     """
@@ -43,6 +43,7 @@ def parse_score(text, tpq=TPQ):
     tick = 0
     bar_ticks = 0
     bar_no = 1
+    saw_sep = False
     for tok in text.split():
         if tok == "|":
             if bar_ticks != tpq * 4:
@@ -50,6 +51,7 @@ def parse_score(text, tpq=TPQ):
                                  % (bar_no, bar_ticks, tpq * 4))
             bar_ticks = 0
             bar_no += 1
+            saw_sep = True
             continue
         if tok.startswith("r"):
             dur = {"r1": tpq * 4, "r2": tpq * 2, "r4": tpq, "r8": tpq // 2,
@@ -69,6 +71,9 @@ def parse_score(text, tpq=TPQ):
         out.append((tick, pitch, dur, vel))
         tick += dur
         bar_ticks += dur
+    if saw_sep and bar_ticks != tpq * 4:
+        raise ValueError("bar %d totals %d ticks, expected %d (4/4)"
+                         % (bar_no, bar_ticks, tpq * 4))
     return out