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"]

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>

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)))

test/clj/auto_ap/ssr/selmer_test.clj

@@ -0,0 +1,36 @@
+(ns auto-ap.ssr.selmer-test
+  (:require
+   [auto-ap.ssr.selmer :as sut]
+   [clojure.string :as str]
+   [clojure.test :refer [deftest is testing]]
+   [hiccup2.core :as h2]))
+
+(deftest hiccup->html
+  (testing "renders a Hiccup form to an HTML string"
+    (is (= "<span class=\"label\">A &amp; B</span>"
+           (sut/hiccup->html [:span.label "A & B"])))))
+
+(deftest selmer-embeds-hiccup
+  (testing "a Hiccup component renders inside a Selmer template via |safe"
+    (let [frag (sut/hiccup->html [:span.badge "from hiccup"])
+          out  (sut/render-str "<div>{{frag|safe}}</div>" {:frag frag})]
+      (is (str/includes? out "<span class=\"badge\">from hiccup</span>"))
+      ;; without |safe the markup would be escaped; |safe keeps it verbatim
+      (is (not (str/includes? out "&lt;span"))))))
+
+(deftest selmer-fragment-inside-hiccup
+  (testing "a Selmer fragment renders inside a Hiccup tree without double-escaping"
+    (let [sel (sut/render-str "<a href=\"{{url}}\">{{label}}</a>" {:url "/x" :label "Go"})
+          out (str (h2/html {} [:div (sut/raw sel)]))]
+      (is (= "<div><a href=\"/x\">Go</a></div>" out)))))
+
+(deftest render-file-from-classpath
+  (testing "render-file resolves a template under resources/templates and keeps plain-HTML Alpine/HTMX attrs"
+    (let [out (sut/render "templates/interop-smoke.html"
+                          {:title "Interop OK"
+                           :hiccup_frag (sut/hiccup->html [:span.badge "from hiccup"])})]
+      (is (str/includes? out "Interop OK"))
+      (is (str/includes? out "from hiccup"))
+      ;; plain-HTML attributes (the whole point of Selmer) survive unambiguously
+      (is (str/includes? out "x-model=\"value.value\""))
+      (is (str/includes? out "tippy?.show()")))))