Drop Options generic and thread signal directly

[manzt]

AsyncReadable<Options> existed so stores could receive arbitrary per-call state. Auth headers, presigning context, cancellation signals, even chunk-layer concerns like caching and prefetch priority (#296, vole-core's wrapArray).

It was a catch-all for extensions that had nowhere else to live, and the cost was a pile of type magic: higher-kinded-type encoding in the store middleware system (#384), threading generic types that didn't actually provide that much type safety. (TypeScript could often bail out to any when inference broke.)

Those extensions now have proper homes. Store middleware (#384) gives transport-layer concerns (auth, presigning, request transformation) a proper extension point, and the custom fetch option on FetchStore (#388) handles the per-store cases at the callsite. Chunk-layer concerns will move to zarr.extendArray in a follow-up.

What's left is signal, which now lives properly on the (non-generic) AsyncReadable interface and can be passed directly in zarr.get and zarr.set from the caller:

// Before
interface AsyncReadable<Options = unknown> {
  get(key: AbsolutePath, opts?: Options): Promise<Uint8Array | undefined>;
}
await zarr.get(arr, null, { opts: { signal: ctl.signal } });

// After
interface AsyncReadable {
  get(key: AbsolutePath, opts?: { signal?: AbortSignal }): Promise<Uint8Array | undefined>;
}
await zarr.get(arr, null, { signal: ctl.signal });

Batched caller signals in withRangeBatching are now merged with AbortSignal.any instead of a user-supplied mergeOptions reducer. The deprecated opts?: { signal? } shape still works for one major version and is folded into the new signal via AbortSignal.any.

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