sx-http: fix navigation + Playwright nav tests — 5/5 pass

AJAX navigation: detect SX-Request/HX-Request headers and return just
the #main-panel fragment instead of the full page shell. Fixes layout
break where header and content appeared side-by-side after navigation.

New navigation test suite (tests/playwright/navigation.spec.js):
- layout stays vertical after clicking nav link
- content updates after navigation
- no raw SX component calls visible after navigation
- header island survives navigation
- full page width is used (no side-by-side split)
All 5 tests pass. 14 total Playwright tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -2052,12 +2052,26 @@ let http_mode port =
     (try Unix.close client with _ -> ())
   in
 
+  (* Check if request has SX-Request or HX-Request header (AJAX navigation) *)
+  let is_sx_request data =
+    let lower = String.lowercase_ascii data in
+    let has_substring s sub =
+      let slen = String.length s and sublen = String.length sub in
+      if sublen > slen then false
+      else let rec check i = if i > slen - sublen then false
+        else if String.sub s i sublen = sub then true else check (i + 1)
+      in check 0
+    in
+    has_substring lower "sx-request" || has_substring lower "hx-request"
+  in
+
   (* Handle one HTTP request *)
   let handle_client env client =
     let buf = Bytes.create 8192 in
     let n = try Unix.read client buf 0 8192 with _ -> 0 in
     if n > 0 then begin
       let data = Bytes.sub_string buf 0 n in
+      let is_ajax = is_sx_request data in
       let response =
         try
           match parse_http_request data with
@@ -2072,18 +2086,50 @@ let http_mode port =
               else
               let is_sx = path = "/sx/" || path = "/sx"
                 || (String.length path > 4 && String.sub path 0 4 = "/sx/") in
-              if is_sx then
-                (* Check cache first *)
+              if is_sx then begin
+                if is_ajax then
+                  (* AJAX navigation — return just the content fragment,
+                     not the full page shell. The client swaps #main-panel. *)
+                  (match http_render_page env path with
+                   | Some html ->
+                     (* Extract #main-panel from the full page HTML *)
+                     let panel_start = try
+                       let idx = ref 0 in
+                       let found = ref false in
+                       while not !found && !idx < String.length html - 20 do
+                         if String.sub html !idx 18 = "id=\"main-panel\"" then
+                           found := true
+                         else
+                           idx := !idx + 1
+                       done;
+                       if !found then begin
+                         (* Walk back to find the opening < *)
+                         let start = ref !idx in
+                         while !start > 0 && html.[!start] <> '<' do
+                           start := !start - 1
+                         done;
+                         Some !start
+                       end else None
+                     with _ -> None in
+                     (match panel_start with
+                      | Some start ->
+                        (* Find matching close tag — scan for </section> or end *)
+                        let fragment = String.sub html start (String.length html - start) in
+                        http_response ~content_type:"text/html; charset=utf-8" fragment
+                      | None -> http_response html)
+                   | None -> http_response ~status:404 "<h1>Not Found</h1>")
+                else
+                (* Full page request — check cache *)
                 match Hashtbl.find_opt response_cache path with
                 | Some cached -> cached
                 | None ->
-                  (* Cache miss — render, cache, return *)
                   (match http_render_page env path with
                    | Some html ->
                      let resp = http_response html in
                      Hashtbl.replace response_cache path resp;
                      resp
                    | None -> http_response ~status:404 "<h1>Not Found</h1>")
+              end
               else if String.length path > 8 && String.sub path 0 8 = "/static/" then
                 serve_static_file static_dir path
               else