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/falado.tex

@@ -10,12 +10,12 @@
 }
 
 \begin{verse}
-F \chord{F}alado, o F \chord{C}alado, \\
-w \chord{F}er sailt m \chord{d}it \chord{B} nach Fala \chord{F}do? \\
-Jeder sucht es, k \chord{C}einer fand \\
-F \chord{d}alado, das W \chord{C}underland. \\
-E \chord{d}in Mast ho, z \chord{A}wei Mast ho, \\
-Dr \chord{d}eimast sailt nach F \chord{A}al \chord{d}ado.
+\chord{F}Falado, o \chord{C}Falado, \\
+\chord{F}wer sailt \chord{d}mit \chord{B}nach \chord{F}Falado? \\
+Jeder sucht es, \chord{C}keiner fand \\
+\chord{d}Falado, das \chord{C}Wunderland. \\
+\chord{d}Ein Mast ho, \chord{A}zwei Mast ho, \\
+\chord{d}Dreimast sailt nach \chord{d}Falado.
 \end{verse}
 
 \begin{verse}