Live demo islands with highlighted source: SSR-safe effect, native component-source

- signals.sx: guard effect body with (when (client?) ...) so effects
  are no-op during SSR — only 2 stubs needed (effect, register-in-scope)
- sx_primitives.ml: add resource SSR stub (returns signal {loading: true}),
  remove 27 unnecessary browser primitive stubs
- sx_server.ml: native component-source that looks up Component/Island
  from env and pretty-prints the definition (replaces broken Python helper)
- reactive-islands/index.sx: Examples section with all 15 live demos
  inline + highlighted source via component-source
- reactive-islands/demo.sx: replace 14 hardcoded highlight strings with
  (component-source "~name") calls for always-current source

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

spec/signals.sx

@@ -109,7 +109,7 @@
       ((deps (list)) (disposed false) (cleanup-fn nil))
       (let
         ((run-effect (fn () (when (not disposed) (when cleanup-fn (cek-call cleanup-fn nil)) (for-each (fn ((dep :as signal)) (signal-remove-sub! dep run-effect)) deps) (set! deps (list)) (let ((ctx (dict "deps" (list) "notify" run-effect))) (scope-push! "sx-reactive" ctx) (let ((result (cek-call effect-fn nil))) (scope-pop! "sx-reactive") (set! deps (get ctx "deps")) (when (callable? result) (set! cleanup-fn result))))))))
-        (run-effect)
+        (when (client?) (run-effect))
         (let
           ((dispose-fn (fn () (set! disposed true) (when cleanup-fn (cek-call cleanup-fn nil)) (for-each (fn ((dep :as signal)) (signal-remove-sub! dep run-effect)) deps) (set! deps (list)))))
           (register-in-scope dispose-fn)