Fix custom mouse event handler semantics

[ShiroKSH]

• Preserve mouseenter and mouseleave semantics when handlers use event namespaces.
• Store semantic event types separately from their backing DOM event types.
• Fix callback removal, remove-all, deduplication, and one-off handler behavior.
• Add regression tests for direct and delegated handlers.
• Verify the full suite with npm test (1,239 tests).