fix: support '.' and 'NC' in --prefix argument

_encode_prefix now handles hold ('.') and no-chord ('NC') tokens
alongside chord symbols, and returns (ids, n_positions) so that
pos_in_bar is tracked correctly regardless of token type.

Fixes ChordParseError when dots were passed in --prefix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_generate.py

@@ -0,0 +1,58 @@
+"""Tests for src/generate.py — prefix encoding and position tracking."""
+
+import pytest
+
+from src.generate import _encode_prefix, _HOLD, _NC, _ROOT_START, _BASS_START, _BASS_END
+from src.tokenizer import TOKEN_TO_ID
+
+
+def test_encode_prefix_chord_only():
+    # "Cmaj7" in C major (shift=0) → ROOT_C QUAL_maj7 EXT_none BASS_root
+    ids, n_pos = _encode_prefix(["Cmaj7"], shift=0)
+    assert len(ids) == 4
+    assert n_pos == 1
+    assert ids[0] == TOKEN_TO_ID["ROOT_C"]
+    assert ids[3] == TOKEN_TO_ID["BASS_root"]
+
+
+def test_encode_prefix_hold():
+    ids, n_pos = _encode_prefix(["."], shift=0)
+    assert ids == [_HOLD]
+    assert n_pos == 1
+
+
+def test_encode_prefix_nc():
+    ids, n_pos = _encode_prefix(["NC"], shift=0)
+    assert ids == [_NC]
+    assert n_pos == 1
+
+
+def test_encode_prefix_mixed():
+    # "Abmaj7 . Fm7 . Db . Bbm ." — what the user tried and got a parse error on
+    symbols = ["Abmaj7", ".", "Fm7", ".", "Db", ".", "Bbm", "."]
+    # Ab major → shift to canonical C major = (0 - 8) % 12 = 4
+    shift = (0 - 8) % 12
+    ids, n_pos = _encode_prefix(symbols, shift=shift)
+    assert n_pos == 8                     # 8 bar positions
+    # Dots become single HOLD tokens; chords become 4-token groups
+    # Total tokens: 4 chords × 4 + 4 holds × 1 = 20
+    assert len(ids) == 20
+    # Every dot position must be a HOLD
+    hold_positions = [1, 3, 5, 7]        # indices in the symbol list
+    # Reconstruct expected token positions: chord at 0→tokens 0-3, dot→token 4, etc.
+    token_idx = 0
+    for sym in symbols:
+        if sym == ".":
+            assert ids[token_idx] == _HOLD
+            token_idx += 1
+        else:
+            assert _ROOT_START <= ids[token_idx] <= TOKEN_TO_ID["ROOT_B"]
+            assert _BASS_START <= ids[token_idx + 3] <= _BASS_END
+            token_idx += 4
+
+
+def test_encode_prefix_position_count_with_holds():
+    # Mixed prefix: 2 chords and 2 holds = 4 positions
+    ids, n_pos = _encode_prefix(["Am", ".", "G", "."], shift=0)
+    assert n_pos == 4
+    assert len(ids) == 2 * 4 + 2 * 1   # 10 tokens