Remove render_to_sx from public API: enforce sx_call for all service code

Replace ~250 render_to_sx calls across all services with sync sx_call,
converting many async functions to sync where no other awaits remained.
Make render_to_sx/render_to_sx_with_env private (_render_to_sx).
Add (post-header-ctx) IO primitive and shared post/post-admin defmacros.
Convert built-in post/post-admin layouts from Python to register_sx_layout
with .sx defcomps. Remove dead post_admin_mobile_nav_sx.

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

test/sxc/pages/renders.py

@@ -5,7 +5,7 @@ import os
 from datetime import datetime
 
 from shared.sx.jinja_bridge import load_service_components
-from shared.sx.helpers import render_to_sx, SxExpr, render_to_sx_with_env, full_page_sx
+from shared.sx.helpers import sx_call, SxExpr, render_to_sx_with_env, full_page_sx
 
 # Load test-specific .sx components at import time
 load_service_components(os.path.dirname(os.path.dirname(os.path.dirname(__file__))))
@@ -75,7 +75,7 @@ def _build_summary_data(result: dict | None, running: bool, csrf: str,
 
 async def test_detail_sx(test: dict) -> str:
     """Return s-expression wire format for a test detail view."""
-    return await render_to_sx("test-detail-section", test=test)
+    return sx_call("test-detail-section", test=test)
 
 
 async def render_dashboard_page_sx(ctx: dict, result: dict | None,
@@ -96,9 +96,9 @@ async def render_dashboard_page_sx(ctx: dict, result: dict | None,
         else:
             summary_data["state"] = "empty-filtered"
 
-    inner = await render_to_sx("test-results-partial",
+    inner = sx_call("test-results-partial",
         summary_data=summary_data, sections=sections, has_failures=has_failures)
-    content = await render_to_sx("test-results-wrap", running=running, inner=SxExpr(inner))
+    content = sx_call("test-results-wrap", running=running, inner=SxExpr(inner))
     hdr = await render_to_sx_with_env("test-layout-full", {},
         services=_service_list(),
         active_service=active_service,
@@ -124,9 +124,9 @@ async def render_results_partial_sx(result: dict | None, running: bool,
         else:
             summary_data["state"] = "empty-filtered"
 
-    inner = await render_to_sx("test-results-partial",
+    inner = sx_call("test-results-partial",
         summary_data=summary_data, sections=sections, has_failures=has_failures)
-    return await render_to_sx("test-results-wrap", running=running, inner=SxExpr(inner))
+    return sx_call("test-results-wrap", running=running, inner=SxExpr(inner))
 
 
 async def render_test_detail_page_sx(ctx: dict, test: dict) -> str:
@@ -136,7 +136,7 @@ async def render_test_detail_page_sx(ctx: dict, test: dict) -> str:
         test_nodeid=test["nodeid"],
         test_label=test["nodeid"].rsplit("::", 1)[-1],
     )
-    content = await render_to_sx("test-detail",
+    content = sx_call("test-detail",
         nodeid=test["nodeid"],
         outcome=test["outcome"],
         duration=str(test["duration"]),