Migrate remaining 7 ref endpoints to SX, add :returns type annotations

Add 14 new IO primitives to boundary.sx: web interop (request-form,
request-json, request-header, request-content-type, request-args-all,
request-form-all, request-headers-all, request-file-name), response
manipulation (set-response-header, set-response-status), ephemeral
state (state-get, state-set!), and timing (now, sleep).

All 19 reference handlers now have :returns type annotations using
types.sx vocabulary. Response meta (headers/status) flows through
context vars, applied by register_route_handlers after execution.

Only SSE endpoint remains in Python (async generator paradigm).

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

shared/sx/types.py

@@ -235,6 +235,7 @@ class HandlerDef:
     path: str | None = None       # public route path (None = internal fragment only)
     method: str = "get"           # HTTP method (get, post, put, patch, delete)
     csrf: bool = True             # CSRF protection enabled
+    returns: str = "element"      # return type (types.sx vocabulary)
 
     @property
     def is_route(self) -> bool: