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/oh-fischer.tex

@@ -7,9 +7,9 @@
 }
 
 \begin{verse}
-/:Oh F \chord{D}ischer, a \chord{A}uf den Wo \chord{D}gen, oh Federi. :/ \\
-/: Ach k \chord{G}omm in deinem Kahn \chord{D}, colla tua bella b \chord{A}arca, \\
-tua bella sene \chord{D}va, federi \chord{A}l \chord{D}inla. :/
+/:Oh \chord{D}Fischer, \chord{A}auf den \chord{D}Wogen, oh Federi. :/ \\
+/: Ach \chord{G}komm in deinem \chord{D}Kahn, colla tua bella \chord{A}barca, \\
+tua bella sene \chord{D}va, federi \chord{D}linla. :/
 \end{verse}
 
 \begin{verse}