Phase 4: Client-side rendering of :data pages via abstract resolve-page-data

Spec layer (orchestration.sx):
- try-client-route now handles :data pages instead of falling back to server
- New abstract primitive resolve-page-data(name, params, callback) — platform
  decides transport (HTTP, IPC, cache, etc)
- Extracted swap-rendered-content and resolve-route-target helpers

Platform layer (bootstrap_js.py):
- resolvePageData() browser implementation: fetches /sx/data/<name>, parses
  SX response, calls callback. Other hosts provide their own transport.

Server layer (pages.py):
- evaluate_page_data() evaluates :data expr, serializes result as SX
- auto_mount_page_data() mounts /sx/data/ endpoint with per-page auth
- _build_pages_sx now computes component deps for all pages (not just pure)

Test page at /isomorphism/data-test exercises the full pipeline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/bootstrap_js.py

@@ -2637,6 +2637,42 @@ PLATFORM_ORCHESTRATION_JS = """
     }
   }
 
+  function resolvePageData(pageName, params, callback) {
+    // Platform implementation: fetch page data via HTTP from /sx/data/ endpoint.
+    // The spec only knows about resolve-page-data(name, params, callback) —
+    // this function provides the concrete transport.
+    var url = "/sx/data/" + encodeURIComponent(pageName);
+    if (params && !isNil(params)) {
+      var qs = [];
+      var ks = Object.keys(params);
+      for (var i = 0; i < ks.length; i++) {
+        var v = params[ks[i]];
+        if (v !== null && v !== undefined && v !== NIL) {
+          qs.push(encodeURIComponent(ks[i]) + "=" + encodeURIComponent(v));
+        }
+      }
+      if (qs.length) url += "?" + qs.join("&");
+    }
+    var headers = { "SX-Request": "true" };
+    fetch(url, { headers: headers }).then(function(resp) {
+      if (!resp.ok) {
+        logWarn("sx:data resolve failed " + resp.status + " for " + pageName);
+        return;
+      }
+      return resp.text().then(function(text) {
+        try {
+          var exprs = parse(text);
+          var data = exprs.length === 1 ? exprs[0] : {};
+          callback(data || {});
+        } catch (e) {
+          logWarn("sx:data parse error for " + pageName + ": " + (e && e.message ? e.message : e));
+        }
+      });
+    }).catch(function(err) {
+      logWarn("sx:data resolve error for " + pageName + ": " + (err && err.message ? err.message : err));
+    });
+  }
+
   function urlPathname(href) {
     try {
       return new URL(href, location.href).pathname;