WASM browser build: interned env keys, async kernel boot, bundled .sx platform

- Symbol interning in sx_types.ml: env lookups use int keys (intern/unintern)
  to avoid repeated string hashing in scope chain walks
- sx-platform.js: poll for SxKernel availability (WASM init is async)
- shell.sx: load sx_browser.bc.wasm.js when SX_USE_WASM=1
- bundle.sh: fix .sx file paths (web-signals.sx rename)
- browser/dune: target byte+js+wasm modes
- Bundle 23 .sx platform files for browser (dom, signals, router, boot, etc.)

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

shared/sx/templates/shell.sx

@@ -90,7 +90,7 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
         (if use-wasm
           (let ((wv (or wasm-hash "dev")))
             (<>
-              (script :src (str asset-url "/wasm/sx_browser.bc.js?v=" wv))
+              (script :src (str asset-url "/wasm/sx_browser.bc.wasm.js?v=" wv))
               (script :src (str asset-url "/wasm/sx-platform.js?v=" wv))))
           (script :src (str asset-url "/scripts/sx-browser.js?v=" sx-js-hash)))
         ;; Body scripts — configurable per app