Add Fretboard.keyboard() for piano/synth controllers

Fretboard.keyboard(keys=88, start="A0") — configurable key count:
- 88 keys = full piano (A0-C8)
- 61 keys = standard synth controller
- 49/37/25 keys = compact MIDI controllers

25 instrument presets total. 368 tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pytheory/chords.py

@@ -927,6 +927,32 @@ class Fretboard:
             Tone.from_string("E4", system="western"),
         ])
 
+    @classmethod
+    def keyboard(cls, keys=88, start="A0"):
+        """Piano or keyboard with the given number of keys.
+
+        Args:
+            keys: Number of keys (default 88 for a full piano).
+                Common sizes: 25, 37, 49, 61, 76, 88.
+            start: The lowest note (default ``"A0"`` for standard piano).
+
+        A full 88-key piano spans A0 (27.5 Hz) to C8 (4186 Hz) —
+        the widest range of any standard acoustic instrument.
+        Smaller MIDI controllers typically start at C.
+
+        Examples::
+
+            Fretboard.keyboard()            # 88-key piano
+            Fretboard.keyboard(61, "C2")    # 61-key controller
+            Fretboard.keyboard(25, "C3")    # 25-key mini controller
+        """
+        from .tones import Tone
+        start_tone = Tone.from_string(start, system="western")
+        tones = []
+        for i in range(keys - 1, -1, -1):
+            tones.append(start_tone.add(i))
+        return cls(tones=tones)
+
     @classmethod
     def lute(cls):
         """Renaissance lute in G tuning (6 courses).

test_pytheory.py

@@ -1942,7 +1942,7 @@ def test_all_instruments_create():
         "violin", "viola", "cello", "double_bass",
         "banjo", "harp", "pedal_steel",
         "bouzouki", "oud", "sitar", "shamisen", "erhu",
-        "charango", "pipa", "balalaika", "lute",
+        "charango", "pipa", "balalaika", "lute", "keyboard",
     ]
     for name in instruments:
         fb = getattr(Fretboard, name)()
@@ -2002,6 +2002,23 @@ def test_fretboard_pipa():
     assert len(fb) == 4
 
 
+def test_keyboard_88():
+    kb = Fretboard.keyboard()
+    assert len(kb) == 88
+
+
+def test_keyboard_25():
+    kb = Fretboard.keyboard(25, "C3")
+    assert len(kb) == 25
+    assert kb.tones[-1].name == "C"
+    assert kb.tones[-1].octave == 3
+
+
+def test_keyboard_custom():
+    kb = Fretboard.keyboard(61, "C2")
+    assert len(kb) == 61
+
+
 # ── Ergonomic integration tests ─────────────────────────────────────────────
 
 def test_ergonomic_workflow():