What the resolver guarantees

Attachments are not concatenated and left to renderer order. The resolver applies explicit rules:

• classes are additive and deduplicated into one Class;
• inline styles from Style pieces merge per property, later attachments winning, over the view's base; a property a Behavior sets (through h.Style in its attributes) has exactly one owner, so a Style piece, the base, or a second Behavior setting the same property is an error;
• an event or scalar attribute has exactly one owner — a second owner is an error;
• a ChildAttribute (a Submodel's published handler) is preserved by identity;
• Key and InnerHTML are structural and cannot be overridden;
• multiple mount contributions compose into exactly one OnMount.

Conflicts throw Diagnostics.DiagnosticError with stable codes such as mixins:event-conflict, mixins:style-property-conflict and mixins:protected-attribute, so tests, DevTools and editor tooling can all speak the same diagnostic language. The style rule exists because a positioning Behavior that writes top and a Style that also sets top would otherwise fight silently, last writer winning.