feat: switch BOM export to a formatted xlsx template instead of generated sheets

Copy a pre-built "BOM Template.xlsx" (with headers/formatting) and write into
it rather than deleting/recreating worksheets from scratch, so output keeps
consistent styling. Cut Templates sheet no longer stores a File Name column;
the DXF filename is now derived from the stored revision, simplifying the
existing-templates map and removing redundant state.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,9 @@
 ## Ignore Visual Studio temporary files, build results, and
 ## files generated by popular Visual Studio add-ons.
 
+# Stray Windows redirect artifact
+/nul
+
 # User-specific files
 *.suo
 *.user