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/ref/primitives.sx

@@ -443,6 +443,16 @@
   :doc "Split comma-separated ID string into list of trimmed non-empty strings.")
 
 
+;; --------------------------------------------------------------------------
+;; Assertions
+;; --------------------------------------------------------------------------
+
+(define-primitive "assert"
+  :params (condition &rest message)
+  :returns "boolean"
+  :doc "Assert condition is truthy; raise error with message if not.")
+
+
 ;; --------------------------------------------------------------------------
 ;; CSSX — style system primitives
 ;; --------------------------------------------------------------------------