Split boundary.sx: separate language contract from app-specific declarations

boundary.sx was mixing three concerns in one file:
- Core SX I/O primitives (the language contract)
- Deployment-specific layout I/O (app architecture)
- Per-service page helpers (fully app-specific)

Now split into three tiers:
1. shared/sx/ref/boundary.sx — core I/O only (frag, query, current-user, etc.)
2. shared/sx/ref/boundary-app.sx — deployment layout contexts (*-header-ctx, *-ctx)
3. {service}/sx/boundary.sx — per-service page helpers

The boundary parser loads all three tiers automatically. Validation error
messages now point to the correct file for each tier.

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

blog/sx/boundary.sx

@@ -0,0 +1,41 @@
+;; Blog service — page helper declarations.
+
+(define-page-helper "editor-data"
+  :params (&key)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "editor-page-data"
+  :params (&key)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "post-admin-data"
+  :params (&key slug)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "post-data-data"
+  :params (&key slug)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "post-preview-data"
+  :params (&key slug)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "post-entries-data"
+  :params (&key slug)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "post-settings-data"
+  :params (&key slug)
+  :returns "dict"
+  :service "blog")
+
+(define-page-helper "post-edit-data"
+  :params (&key slug)
+  :returns "dict"
+  :service "blog")