Enforce SX boundary contract via boundary.sx spec + runtime validation

Add boundary.sx declaring all 34 I/O primitives, 32 page helpers, and 9
allowed boundary types. Runtime validation in boundary.py checks every
registration against the spec — undeclared primitives/helpers crash at
startup with SX_BOUNDARY_STRICT=1 (now set in both dev and prod).

Key changes:
- Move 5 I/O-in-disguise primitives (app-url, asset-url, config,
  jinja-global, relations-from) from primitives.py to primitives_io.py
- Remove duplicate url-for/route-prefix from primitives.py (already in IO)
- Fix parse-datetime to return ISO string instead of raw datetime
- Add datetime→isoformat conversion in _convert_result at the edge
- Wrap page helper return values with boundary type validation
- Replace all SxExpr(f"...") patterns with sx_call() or _sx_fragment()
- Add assert declaration to primitives.sx

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

sx/sxc/pages/docs.sx

@@ -368,6 +368,10 @@
   :data (bootstrapper-data slug)
   :content (if bootstrapper-not-found
     (~spec-not-found :slug slug)
-    (~bootstrapper-js-content
-      :bootstrapper-source bootstrapper-source
-      :bootstrapped-output bootstrapped-output)))
+    (if (= slug "python")
+      (~bootstrapper-py-content
+        :bootstrapper-source bootstrapper-source
+        :bootstrapped-output bootstrapped-output)
+      (~bootstrapper-js-content
+        :bootstrapper-source bootstrapper-source
+        :bootstrapped-output bootstrapped-output))))

sx/sxc/pages/helpers.py

@@ -135,29 +135,44 @@ def _bootstrapper_data(target: str) -> dict:
     """
     import os
 
-    if target != "javascript":
+    if target not in ("javascript", "python"):
         return {"bootstrapper-not-found": True}
 
     ref_dir = os.path.join(os.path.dirname(__file__), "..", "..", "shared", "sx", "ref")
     if not os.path.isdir(ref_dir):
         ref_dir = "/app/shared/sx/ref"
 
-    # Read bootstrapper source
-    bs_path = os.path.join(ref_dir, "bootstrap_js.py")
-    try:
-        with open(bs_path, encoding="utf-8") as f:
-            bootstrapper_source = f.read()
-    except FileNotFoundError:
-        bootstrapper_source = "# bootstrapper source not found"
+    if target == "javascript":
+        # Read bootstrapper source
+        bs_path = os.path.join(ref_dir, "bootstrap_js.py")
+        try:
+            with open(bs_path, encoding="utf-8") as f:
+                bootstrapper_source = f.read()
+        except FileNotFoundError:
+            bootstrapper_source = "# bootstrapper source not found"
 
-    # Run the bootstrap to generate JS
-    from shared.sx.ref.bootstrap_js import compile_ref_to_js
-    try:
-        bootstrapped_output = compile_ref_to_js(
-            adapters=["dom", "engine", "orchestration", "boot", "cssx"]
-        )
-    except Exception as e:
-        bootstrapped_output = f"// bootstrap error: {e}"
+        # Run the bootstrap to generate JS
+        from shared.sx.ref.bootstrap_js import compile_ref_to_js
+        try:
+            bootstrapped_output = compile_ref_to_js(
+                adapters=["dom", "engine", "orchestration", "boot", "cssx"]
+            )
+        except Exception as e:
+            bootstrapped_output = f"// bootstrap error: {e}"
+
+    elif target == "python":
+        bs_path = os.path.join(ref_dir, "bootstrap_py.py")
+        try:
+            with open(bs_path, encoding="utf-8") as f:
+                bootstrapper_source = f.read()
+        except FileNotFoundError:
+            bootstrapper_source = "# bootstrapper source not found"
+
+        from shared.sx.ref.bootstrap_py import compile_ref_to_py
+        try:
+            bootstrapped_output = compile_ref_to_py()
+        except Exception as e:
+            bootstrapped_output = f"# bootstrap error: {e}"
 
     return {
         "bootstrapper-not-found": None,
@@ -176,7 +191,7 @@ def _attr_detail_data(slug: str) -> dict:
     - attr-not-found (truthy if not found)
     """
     from content.pages import ATTR_DETAILS
-    from shared.sx.helpers import SxExpr
+    from shared.sx.helpers import sx_call
 
     detail = ATTR_DETAILS.get(slug)
     if not detail:
@@ -193,7 +208,7 @@ def _attr_detail_data(slug: str) -> dict:
         "attr-description": detail["description"],
         "attr-example": detail["example"],
         "attr-handler": detail.get("handler"),
-        "attr-demo": SxExpr(f"(~{demo_name})") if demo_name else None,
+        "attr-demo": sx_call(demo_name) if demo_name else None,
         "attr-wire-id": wire_id,
     }
 
@@ -201,7 +216,7 @@ def _attr_detail_data(slug: str) -> dict:
 def _header_detail_data(slug: str) -> dict:
     """Return header detail data for a specific header slug."""
     from content.pages import HEADER_DETAILS
-    from shared.sx.helpers import SxExpr
+    from shared.sx.helpers import sx_call
 
     detail = HEADER_DETAILS.get(slug)
     if not detail:
@@ -214,14 +229,14 @@ def _header_detail_data(slug: str) -> dict:
         "header-direction": detail["direction"],
         "header-description": detail["description"],
         "header-example": detail.get("example"),
-        "header-demo": SxExpr(f"(~{demo_name})") if demo_name else None,
+        "header-demo": sx_call(demo_name) if demo_name else None,
     }
 
 
 def _event_detail_data(slug: str) -> dict:
     """Return event detail data for a specific event slug."""
     from content.pages import EVENT_DETAILS
-    from shared.sx.helpers import SxExpr
+    from shared.sx.helpers import sx_call
 
     detail = EVENT_DETAILS.get(slug)
     if not detail:
@@ -233,5 +248,5 @@ def _event_detail_data(slug: str) -> dict:
         "event-title": slug,
         "event-description": detail["description"],
         "event-example": detail.get("example"),
-        "event-demo": SxExpr(f"(~{demo_name})") if demo_name else None,
+        "event-demo": sx_call(demo_name) if demo_name else None,
     }