search: make the navigation form work from every page (AUD-151)

The nav form carried hx-get="/search" hx-target="#search-results", but
#search-results exists only on /search. On every other page htmx found no
target and silently discarded the response: a search box that looks like it
works and does nothing.

- SearchForm is now a plain GET with no htmx attributes (navigation and the
  landing page), so it navigates to /search?q=… from any page and with
  JavaScript disabled.
- SearchFormLive keeps the in-place swap on the search page, where the target
  does exist. For HX-Request the server now returns the block's *contents*
  (SearchResultsBody), so the fragment no longer nests a second element with
  id="search-results" inside the live region.
- Empty query shows a prompt instead of an empty list. A zero-match query says
  "Nothing found" and offers the six areas plus the evidence index.
- The result count is role="status" inside the existing aria-live container, so
  a screen reader reads "4 results for …" rather than a run of titles.

Tests (internal/server, internal/site):
- the nav form carries no htmx attribute on nine pages, and no built page
  carries an hx-target whose element is missing (walks all of dist/);
- search submitted from /, /edge/ and /platform/ lands on a page with results
  with no HX-Request header, i.e. the JavaScript-disabled path;
- empty query, zero match and the announced count;
- a content change followed by a build is visible in dist/search.db. That is
  the deploy path: deploy/deploy.sh runs `make dist` before it stages a
  release, and search.Build removes and rewrites the index every build.

DoD: search from home, /edge and /platform lands on results; works with
JavaScript disabled; zero-match query shows a message; content change followed
by a deploy is reflected in search; no htmx request targets a missing element.

(cherry picked from commit a56c795768e1e9b10204aae04c3612d7a96775a7)

internal/server/server.go

@@ -413,9 +413,10 @@ func (s *Server) fragmentNotFound(w http.ResponseWriter, msg string) {
 	_, _ = io.WriteString(w, `<div class="stage-empty"><p class="stage-empty-title">`+msg+`</p><p class="stage-hint">The <a href="/evidence/">evidence index</a> lists every fragment in this build.</p></div>`)
 }
 
-// handleSearch runs a keyword search. With HTMX it returns just the results
-// block; without JavaScript the same handler returns the whole page, so the form
-// works in both cases.
+// handleSearch runs a keyword search. With HTMX it returns just the contents of
+// the results block, which the search page's own form swaps into #search-results;
+// without JavaScript the same handler returns the whole page, so the form works
+// in both cases.
 func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 	q := strings.TrimSpace(r.URL.Query().Get("q"))
 	failed := false
@@ -451,7 +452,9 @@ func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 		Render(context.Context, io.Writer) error
 	} = templates.SearchPage(view)
 	if r.Header.Get("HX-Request") == "true" {
-		c = templates.SearchResults(view)
+		// The element that holds #search-results stays where it is, so the
+		// fragment carries the contents and not the wrapper.
+		c = templates.SearchResultsBody(view)
 	}
 	if err := c.Render(r.Context(), &buf); err != nil {
 		s.log.Error("render search", "error", err)