Fix client routing: fall through to server on layout/section change

Client-side routing was only swapping #main-panel content without
updating OOB headers (nav rows, sub-rows). Now each page entry in the
registry includes a layout identity (e.g. "sx-section:Testing") and
try-client-route falls through to server when layout changes, so OOB
header updates are applied correctly.

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

shared/sx/helpers.py

@@ -704,6 +704,31 @@ def _build_pages_sx(service: str) -> str:
             if io_deps else "()"
         )
 
+        # Extract layout identity for client-side routing.
+        # When layout changes between pages, client routing falls through
+        # to server so OOB header updates are applied.
+        layout_id = ""
+        if isinstance(page_def.layout, str):
+            layout_id = page_def.layout
+        elif isinstance(page_def.layout, list):
+            from .types import Keyword as _Kw, Symbol as _Sym
+            first = page_def.layout[0]
+            if isinstance(first, _Kw):
+                layout_id = first.name
+            elif isinstance(first, _Sym):
+                layout_id = first.name
+            else:
+                layout_id = str(first)
+            # Append section kwarg to distinguish same-layout-type
+            # with different sections (e.g. sx-section+Docs vs sx-section+Testing)
+            raw_layout = page_def.layout
+            for li in range(1, len(raw_layout) - 1):
+                if isinstance(raw_layout[li], _Kw) and raw_layout[li].name == "section":
+                    val = raw_layout[li + 1]
+                    if val is not None:
+                        layout_id = f"{layout_id}:{val}"
+                    break
+
         # Build closure as SX dict
         closure_parts: list[str] = []
         for k, v in page_def.closure.items():
@@ -719,6 +744,7 @@ def _build_pages_sx(service: str) -> str:
             + " :auth " + _sx_literal(auth)
             + " :has-data " + has_data
             + " :stream " + stream
+            + " :layout " + _sx_literal(layout_id)
             + " :io-deps " + io_deps_sx
             + " :content " + _sx_literal(content_src)
             + " :deps " + deps_sx