PiDirective lowercases the PI target, dropping case in violation of the XML spec

The constructor of PiDirective lowercases the target name with tgt.toLowerCase(Locale.ENGLISH) before storing it in this.target, see src/main/java/org/xembly/PiDirective.java around line 38. The lowercased value is later used in Document.createProcessingInstruction(this.target.raw(), this.data.raw()) around line 60, so the case information from the caller is lost in the produced DOM.

XML 1.0 Names are case-sensitive, and the PITarget production in section 2.6 inherits that. Calling new Directives().pi("MyApp", "x") ends up emitting <?myapp x?> instead of <?MyApp x?>, which is observably different from what every other XML toolchain produces for the same input. Callers that expect the target they wrote to round-trip through xembly silently get a different document.

The lowercasing looks like it was meant to detect the reserved xml prefix (case-insensitive per the spec), but storing the lowercased form goes further than that and rewrites the user input. The fix is to keep tgt as-is in this.target and, if a check is still wanted, reject targets that match (?i)^xml$ instead of folding the case.

Start in src/main/java/org/xembly/PiDirective.java around lines 38 and 60, then reproduce the issue with new Directives().pi("MyApp", "x"). Verify that the processing-instruction target preserves the caller's case and that reserved xml-target handling remains correct; the emitted result should use MyApp rather than myapp.