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>

shared/sx/primitives.py

@@ -30,6 +30,8 @@ def register_primitive(name: str):
             return "".join(str(a) for a in args)
     """
     def decorator(fn: Callable) -> Callable:
+        from .boundary import validate_primitive
+        validate_primitive(name)
         _PRIMITIVES[name] = fn
         return fn
     return decorator
@@ -431,60 +433,6 @@ def prim_into(target: Any, coll: Any) -> Any:
     raise ValueError(f"into: unsupported target type {type(target).__name__}")
 
 
-# ---------------------------------------------------------------------------
-# URL helpers
-# ---------------------------------------------------------------------------
-
-@register_primitive("app-url")
-def prim_app_url(service: str, path: str = "/") -> str:
-    """``(app-url "blog" "/my-post/")`` → full URL for service."""
-    from shared.infrastructure.urls import app_url
-    return app_url(service, path)
-
-
-@register_primitive("url-for")
-def prim_url_for(endpoint: str, **kwargs: Any) -> str:
-    """``(url-for "endpoint")`` → quart.url_for."""
-    from quart import url_for
-    return url_for(endpoint, **kwargs)
-
-
-@register_primitive("asset-url")
-def prim_asset_url(path: str = "") -> str:
-    """``(asset-url "/img/logo.png")`` → versioned static URL."""
-    from shared.infrastructure.urls import asset_url
-    return asset_url(path)
-
-
-@register_primitive("config")
-def prim_config(key: str) -> Any:
-    """``(config "key")`` → shared.config.config()[key]."""
-    from shared.config import config
-    cfg = config()
-    return cfg.get(key)
-
-
-@register_primitive("jinja-global")
-def prim_jinja_global(key: str, default: Any = None) -> Any:
-    """``(jinja-global "key")`` → current_app.jinja_env.globals[key]."""
-    from quart import current_app
-    return current_app.jinja_env.globals.get(key, default)
-
-
-@register_primitive("relations-from")
-def prim_relations_from(entity_type: str) -> list[dict]:
-    """``(relations-from "page")`` → list of RelationDef dicts."""
-    from shared.sx.relations import relations_from
-    return [
-        {
-            "name": d.name, "from_type": d.from_type, "to_type": d.to_type,
-            "cardinality": d.cardinality, "nav": d.nav,
-            "nav_icon": d.nav_icon, "nav_label": d.nav_label,
-        }
-        for d in relations_from(entity_type)
-    ]
-
-
 # ---------------------------------------------------------------------------
 # Format helpers
 # ---------------------------------------------------------------------------
@@ -520,11 +468,15 @@ def prim_parse_int(val: Any, default: Any = 0) -> int | Any:
 
 @register_primitive("parse-datetime")
 def prim_parse_datetime(val: Any) -> Any:
-    """``(parse-datetime "2024-01-15T10:00:00")`` → datetime object."""
+    """``(parse-datetime "2024-01-15T10:00:00")`` → ISO string or nil."""
     from datetime import datetime
     if not val or val is NIL:
         return NIL
-    return datetime.fromisoformat(str(val))
+    try:
+        dt = datetime.fromisoformat(str(val))
+        return dt.isoformat()
+    except (ValueError, TypeError):
+        return NIL
 
 
 @register_primitive("split-ids")
@@ -570,13 +522,6 @@ def prim_escape(s: Any) -> str:
     return str(_escape(str(s) if s is not None and s is not NIL else ""))
 
 
-@register_primitive("route-prefix")
-def prim_route_prefix() -> str:
-    """``(route-prefix)`` → service URL prefix for dev/prod routing."""
-    from shared.utils import route_prefix
-    return route_prefix()
-
-
 # ---------------------------------------------------------------------------
 # Style primitives
 # ---------------------------------------------------------------------------