perf: cache compiled XSLT Templates in MessageCodeGenerator

[Copilot]

Ports performance optimization from quickfixj#1231 now that the code generator has moved to this repository.

createTransformer() was recompiling XSL stylesheets from scratch on every invocation. For FIX Latest with hundreds of files, the same handful of stylesheets were compiled hundreds of times.

Changes

• Shared TransformerFactory: Moved from local variable to instance field
• Templates cache: ConcurrentHashMap<String, Templates> keyed by absolute path (filesystem XSL) or "classpath:" + xsltFile (embedded resources)
• Lazy compilation: First call per stylesheet compiles via newTemplates(), subsequent calls reuse cached Templates and call cheap newTransformer()
• StreamSource handling: Created inside computeIfAbsent lambda to avoid stream consumption issues in edge cases

// Before: full recompilation per file
TransformerFactory tf = new net.sf.saxon.TransformerFactoryImpl();
return tf.newTransformer(styleSource);  // expensive

// After: compile once, reuse
Templates templates = templatesCache.computeIfAbsent(cacheKey,
    k -> transformerFactory.newTemplates(styleSource));
return templates.newTransformer();  // cheap