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.

project.clj

@@ -96,6 +96,7 @@
                   [org.clojure/core.async]]
 
                  [hiccup "2.0.0-alpha2"]
+                 [selmer "1.12.61"]
 
                  ;; needed for java 11
                  [javax.xml.bind/jaxb-api "2.4.0-b180830.0359"]