Stepper persistence: use def-store instead of broken cookies

The home stepper's step-idx signal was not persisting across SX
navigation because set-cookie/freeze-to-sx wasn't working in the
WASM kernel. Replace with def-store which uses a global registry
that survives island re-hydration.

Also fix sx_http.exe build: add sx_http back to dune, inline scope
primitives (Sx_scope module was removed), add declarative form
stubs and render stubs, fix /sx/ home route mapping.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/dune

@@ -1,5 +1,5 @@
 (executables
- (names run_tests debug_set sx_server integration_tests)
+ (names run_tests debug_set sx_server sx_http integration_tests)
  (libraries sx unix))
 
 (executable