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/sx/nav-data.sx

@@ -97,7 +97,8 @@
 
 (define bootstrappers-nav-items (list
   (dict :label "Overview" :href "/bootstrappers/")
-  (dict :label "JavaScript" :href "/bootstrappers/javascript")))
+  (dict :label "JavaScript" :href "/bootstrappers/javascript")
+  (dict :label "Python" :href "/bootstrappers/python")))
 
 ;; Spec file registry — canonical metadata for spec viewer pages.
 ;; Python only handles file I/O (read-spec-file); all metadata lives here.