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/come-by-the-hills.tex

@@ -6,14 +6,14 @@
 }
 
 \begin{verse}
-Co \chord{D}me by the hi \chord{G}lls to the l \chord{D}and / where f \chord{G}ancy is fr \chord{D}ee. \\
+\chord{D}Come by the \chord{G}hills to the \chord{D}land / where \chord{G}fancy is \chord{D}free. \\
 D                 G              fis \\
 And stand where the peaks meet the sky / and the \\
-lo \chord{e}ughs meet the se \chord{A}a / where the ri \chord{D}vers run c \chord{D7}lear and \\
-the br \chord{G}acken is g \chord{D}old in the s \chord{A}un. \\
+\chord{e}loughs meet the \chord{A}sea / where the \chord{D}rivers run \chord{D7}clear and \\
+the \chord{G}bracken is \chord{D}gold in the \chord{A}sun. \\
 Ref.: \\
-And the care \chord{h}s of tomor \chord{G}row must w \chord{D}ait \\
-until t \chord{G}his day is d \chord{D}on \chord{A}e.
+And the \chord{h}cares of \chord{G}tomorrow must \chord{D}wait \\
+until \chord{G}this day is \chord{A}done.
 \end{verse}
 
 \begin{verse}