Fix chord alignment: snap to word boundaries

Improve merge_chord_lyric() to snap chord positions to the start
of the word they fall within, instead of splitting words mid-way.
Fixes artifacts like "Liebespaar \chord{C}e" → "\chord{C}Liebespaare".

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

songs/dort-an-dem-ueferchen.tex

@@ -11,7 +11,7 @@
 \begin{verse}
 D            A              h                 Fis \\
 Dort an dem Üferchen, entlang dem Fluss Kasanka \\
-\chord{G}ein graubla \chord{A}uer Ent \chord{D}erich schwamm. Heidul \chord{D}i, heid \chord{A}uli \\
+\chord{G}ein \chord{A}graublauer \chord{D}Enterich schwamm. \chord{D}Heiduli, \chord{A}heiduli \\
 h       Fis       G        A     D \\
 heiduli, heiduli, ein graublauer Enterich schwamm.
 \end{verse}