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/whats-right-ye-jacobites.tex

@@ -9,16 +9,16 @@
 }
 
 \begin{verse}
-What’s r \chord{e}ight and what is wrong by the l \chord{G}aw, by the l \chord{D}aw? \\
-What’s r \chord{e}ight and what is wrong \chord{D}by the l \chord{e}aw? \\
-\chord{D}What’s \chord{G}right and what is wrong by s \chord{D}hort or by long? \\
-A we \chord{e}ak arm or a strong f \chord{D}or to d \chord{e}raw, for to d \chord{D}raw? \\
-A we \chord{e}ak arm or a strong f \chord{D}or to d \chord{e}raw. \\
-Ye J \chord{e}acobites by name lend an e \chord{G}ar, lend an \chord{D}ear. \\
-Ye \chord{e}Jacobites by name l \chord{D}end an e \chord{e}ar. \\
-\chord{D}Ye \chord{G} Jacobites by name, your f \chord{D}aults I will proclaim, \\
-Your d \chord{e}octrines I must blame you shall h \chord{G}ear, you shall h \chord{D}ear. \\
-Your d \chord{e}octrines I must blame y \chord{D}ou shall h \chord{e}ear.
+What’s \chord{e}right and what is wrong by the \chord{G}law, by the \chord{D}law? \\
+What’s \chord{e}right and what is wrong \chord{D}by the \chord{e}law? \\
+\chord{D}What’s \chord{G}right and what is wrong by \chord{D}short or by long? \\
+A \chord{e}weak arm or a strong \chord{D}for to \chord{e}draw, for to \chord{D}draw? \\
+A \chord{e}weak arm or a strong \chord{D}for to \chord{e}draw. \\
+Ye \chord{e}Jacobites by name lend an \chord{G}ear, lend an \chord{D}ear. \\
+Ye \chord{e}Jacobites by name \chord{D}lend an \chord{e}ear. \\
+\chord{D}Ye \chord{G}Jacobites by name, your \chord{D}faults I will proclaim, \\
+Your \chord{e}doctrines I must blame you shall \chord{G}hear, you shall \chord{D}hear. \\
+Your \chord{e}doctrines I must blame \chord{D}you shall \chord{e}hear.
 \end{verse}
 
 \begin{verse}