Step 10c: fix bind subscriber re-evaluation — track names not frames

Root cause: context called inside lambdas (e.g. swap!) went through
nested cek_run with empty kont, so provide frames weren't found and
never tracked to *bind-tracking*.

Three changes in evaluator.sx:
- step-sf-context: track context names (not frames) to *bind-tracking*
  — names work across cek_run boundaries via scope-peek fallback
- bind continue: resolve tracked names to frames via kont-find-provide
  on rest-k before registering subscribers
- subscriber: use empty kont instead of kont-extract-provides — old
  approach created provide frames whose continue handlers called
  scope-pop!, corrupting the scope stack

2752/2768 OCaml tests pass (all 7 bind subscriber tests fixed).
32/32 WASM native tests pass.

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-418217b8",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-0734e8ba",[2,3,5]],["std_exit-10fb8830",[2]],["start-f5d3f095",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-b2cc3269",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-9b135b3a",[2,3,5]],["std_exit-10fb8830",[2]],["start-f5d3f095",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new