fix: apply transformTagName once to self-closing tags

[youdie006]

Purpose / Goal

Self-closing tags go through transformTagName twice: once before the trailing / is stripped, so the callback receives e.g. a/, and again in the self-closing branch. Lowercase or map transforms hide it. With a non-idempotent one they don't:

new XMLParser({ transformTagName: n => 'ns_' + n }).parse('<root><a></a><a/></root>')
// actual:   {"ns_root":{"ns_a":"","ns_ns_a":""}}
// expected: {"ns_root":{"ns_a":["",""]}}

This strips the / before the single transform call and removes the second call, so each tag is transformed once with its real name. Two side effects of the move:

• strictReservedNames now also rejects <#text/>. Before, it produced {"r":[]}, while <#text></#text> and <#text a="1"/> already threw.
• With a non-idempotent transform and allowBooleanAttributes, <a a/> no longer keeps @_a. It only had it because the doubled name made tagName !== tagExp; without a transform it is already dropped. That is a separate bug in the attribute check, not touched here.

Added a spec covering paired, empty and attributed self-closing tags. npm test passes (333 specs) on Node 22 and 14.

Type

• Bug Fix
• Refactoring / Technology upgrade
• New Feature

Written with AI assistance (Claude); I have reviewed the change.