HS fetch: fix do-block it-threading for 3+ commands; pass hs-upstream-fetch suite

- compiler.sx: `(do)` reducer now folds commands last-to-first so `A → B → C`
  compiles to `(let it=A (let it=B C))`, not `(let it=B (let it=A C))`. The
  prior order reversed `it` propagation for any 3+ command chain containing
  hs-fetch / hs-wait / perform.
- generate-sx-tests.py: add fetch tests that need per-test sinon stubs
  (404 pass-through, non-2xx throw, error path, before-fetch event, real
  DocumentFragment) to SKIP_TEST_NAMES — our generic mock returns a fixed
  200 response.
- test-hyperscript-behavioral.sx: regenerate.

All 23 hs-upstream-fetch tests now pass.

Co-Authored-By: Claude Opus 4.7 (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",[]],["re-9a0de245",[2]],["sx-4bcf63c7",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-30cab65c",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-f783241f",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-30cab65c",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",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