feat(ssr): add Selmer dependency + Hiccup<->Selmer interop bridge (Phase 2 foundation)

The strangler foundation for migrating interactive SSR components from Hiccup to
Selmer (plain-HTML Alpine/HTMX attributes instead of mixed keyword/string encodings).

- project.clj: add [selmer "1.12.61"].
- auto-ap.ssr.selmer: render / render-str (selmer/render-file + string), hiccup->html
  (Hiccup -> string for {{ frag|safe }}), raw (wrap a rendered fragment for embedding
  in a Hiccup tree without double-escaping), render->hiccup.
- resources/templates/interop-smoke.html: proves render-file from the classpath and
  that plain-HTML alpine attrs (x-model, @keydown, tippy?.show()) pass through verbatim.
- selmer_test: 4 tests / 8 assertions covering both interop directions; all green.

Proven via REPL + tests: a Hiccup component renders inside a Selmer template, and a
Selmer fragment renders inside a Hiccup tree. Both valid during the transition.

resources/templates/interop-smoke.html

@@ -0,0 +1,7 @@
+<div id="interop-smoke" class="p-2">
+  <h3>{{ title }}</h3>
+  {# a Hiccup-rendered component, passed in pre-rendered and emitted verbatim #}
+  {{ hiccup_frag|safe }}
+  <input x-ref="input" x-model="value.value"
+         @keydown.down.prevent.stop="tippy?.show()" />
+</div>