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/helpers.py

@@ -684,8 +684,10 @@ def _build_pages_sx(service: str) -> str:
         has_data = "true" if page_def.data_expr is not None else "false"
 
         # Component deps needed to render this page client-side
+        # Compute for all pages (including :data pages) — the client
+        # renders both pure and data pages after fetching data
         deps: set[str] = set()
-        if content_src and page_def.data_expr is None:
+        if content_src:
             deps = components_needed(content_src, _COMPONENT_ENV)
         deps_sx = "(" + " ".join(_sx_literal(d) for d in sorted(deps)) + ")"