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>

sx/sx/boundary.sx

@@ -0,0 +1,51 @@
+;; SX docs service — page helper declarations.
+
+(define-page-helper "highlight"
+  :params (code lang)
+  :returns "sx-source"
+  :service "sx")
+
+(define-page-helper "primitives-data"
+  :params ()
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "special-forms-data"
+  :params ()
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "reference-data"
+  :params (slug)
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "attr-detail-data"
+  :params (slug)
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "header-detail-data"
+  :params (slug)
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "event-detail-data"
+  :params (slug)
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "read-spec-file"
+  :params (filename)
+  :returns "string"
+  :service "sx")
+
+(define-page-helper "bootstrapper-data"
+  :params (target)
+  :returns "dict"
+  :service "sx")
+
+(define-page-helper "bundle-analyzer-data"
+  :params ()
+  :returns "dict"
+  :service "sx")