feat(serve-htmx): add HtmlContent terminal send, decouple from base-template

[deer]

Adds HtmlContent — a @FunctionalInterface in serve-template that produces a rendered HTML string — and wires it into HtmxResponse.send(HtmlContent) as the terminal step of the fluent chain. This removes the direct dependency on base.build's base-template module that was introduced when the send overload was first drafted. serve-htmx now depends on serve-template (transitive, so HtmlContent is on callers' module path automatically) and is otherwise ignorant of any specific template engine. Callers using base-template templates adapt with a one-liner () -> { var out = new HtmlOut(); t.render(out); return out.toString(); }, which belongs in the module that knows about that engine. Tests cover the status/content-type/body contract and the preservation of HX-* headers set before the terminal send.