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.

src/clj/auto_ap/ssr/selmer.clj

@@ -0,0 +1,43 @@
+(ns auto-ap.ssr.selmer
+  "Selmer rendering + the Hiccup<->Selmer interop bridge for the SSR form/wizard
+   migration (see .claude/skills/ssr-form-migration). Interactive, attribute-heavy
+   components render from Selmer templates with plain-HTML Alpine/HTMX attributes;
+   the bridge lets a Selmer template embed Hiccup output and lets a Selmer fragment
+   sit inside a Hiccup tree during the strangler transition.
+
+   Templates live under resources/templates/ and are referenced by classpath-relative
+   path, e.g. (render \"templates/components/typeahead.html\" ctx)."
+  (:require
+   [hiccup.util :as hu]
+   [hiccup2.core :as h2]
+   [selmer.parser :as selmer]))
+
+(defn hiccup->html
+  "Render a Hiccup form to an HTML string so it can be embedded in a Selmer
+   context value and emitted with the |safe filter: {{ frag|safe }}."
+  [hiccup]
+  (str (h2/html {} hiccup)))
+
+(defn raw
+  "Wrap an already-rendered HTML string (e.g. from `render`) so hiccup2 emits it
+   verbatim instead of escaping it. Use to drop a Selmer fragment into a Hiccup tree:
+   [:div (sel/raw (sel/render \"...\" ctx))]."
+  [^String html]
+  (hu/raw-string html))
+
+(defn render
+  "Render a Selmer template file (classpath-relative path) with `ctx`, returning an
+   HTML string. Hiccup values in `ctx` should be pre-rendered via `hiccup->html` and
+   referenced with |safe in the template."
+  [template ctx]
+  (selmer/render-file template ctx))
+
+(defn render-str
+  "Render a Selmer template given as a string (handy for tests/REPL)."
+  [template ctx]
+  (selmer/render template ctx))
+
+(defn render->hiccup
+  "Render a Selmer template file and wrap the result for safe embedding in Hiccup."
+  [template ctx]
+  (raw (render template ctx)))