webcore(Blob): coalesce concurrent fd-backed blob reads onto a single ReadFile - #35832

The "JS-thread-only" invariant on `Store::in_flight_blob_reader` is unenforced: `ObjectURLRegistry::singleton()` is a process-global `static OnceLock`, and `resolve_and_dupe` hands back a Blob whose `StoreRef` points to the **same heap `Store`** — so `URL.createObjectURL(Bun.file(fd))` on the main thread + `resolveObjectURL(url)` in a Worker gives both JS threads an fd-backed Store on which `try_coalesce_fd_read`/`mark_in_flight` are live. A worker's `load(Acquire)` can then race main's `then()`

The sliced-vs-unsliced test can flake with `EPIPE: broken pipe, write` at `await proc.stdin.end()` (line 65 of the shared `run()` helper). 938ed8d relaxed the child-side `result[0]` assertion because either reader can lose the `epoll_ctl(ADD)` race, but when the *unsliced* reader is the loser the child exits after consuming ~3 bytes of the 2 MiB payload, and the parent's `FileSink::on_attached_process_exit` rejects the still-pending `end()` promise before any assertion runs. Swallow the rejectio