feat(ruby): QR/Barcode merge placeholders in SVG templates

Add data-kb-placeholder qr/barcode support with rqrcode and barby.
Scale QR to fit placeholder box (viewBox + meet) and barcode by height
with natural width. Declare barby and rqrcode runtime dependencies.

Made-with: Cursor

ruby/libptouch.gemspec

@@ -29,5 +29,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "ffi", "~> 1.15"
+  spec.add_dependency "barby"
+  spec.add_dependency "rqrcode"
   spec.add_dependency "rexml"
 end