Store primitives + event-bridge + client? for isomorphic nav

- def-store/use-store/clear-stores: OCaml primitives with global
  mutable registry. Bypasses env scoping issues that prevented SX-level
  stores from persisting across bytecode module boundaries.

- client? primitive: _is_client ref (false on server, true in browser).
  Registered in primitives table for CALL_PRIM compatibility.

- Event-bridge island: rewritten to use document-level addEventListener
  via effect + host-callback, fixing container-ref timing issue.

- Header island: uses def-store for idx/shade signals when client? is
  true, plain signals when false (SSR compatibility).

- web-signals.sx: SX store definitions removed, OCaml primitives replace.

Isomorphic nav still fixme — client? works from K.eval but the JIT
"Not callable: nil" bug prevents proper primitive resolution during
render-to-dom hydration. Needs JIT investigation.

100 passed, 1 skipped, 0 failed.

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

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-951e6734",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-6b156118",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-6f7dfa09",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-6b156118",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
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 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new