response.blob() drops body bytes — same shape as #227, needs Blob instance methods #234

Summary

Follow-up to #227. response.blob() (in crates/perry-stdlib/src/fetch.rs, immediately below the arrayBuffer() fix that landed via #232) has the same metadata-only-stub bug: resp.body is consumed only for .len(), then the resolved value is a 2-field stub object { size, type } with no actual byte storage. Any user code trying to read bytes out of a Blob sees nothing.

Why this is broader scope than #227

response.arrayBuffer() could be fixed by returning a real BufferHeader because the user-side consumers (new Uint8Array(buf) / Buffer.from(buf)) are already wired through the buffer registry. Blob is a distinct JS type with its own surface that needs to land first:

• blob.arrayBuffer(): Promise<ArrayBuffer>
• blob.text(): Promise<string>
• blob.bytes(): Promise<Uint8Array> (newer, but increasingly common)
• blob.slice(start?, end?, type?): Blob
• blob.stream(): ReadableStream (separate gap; depends on stream impl)

Each method needs:

1. A runtime symbol exported from perry-stdlib (probably keyed off a BLOB_REGISTRY similar to FETCH_RESPONSES).
2. A codegen dispatch row in crates/perry-codegen/src/lower_call.rs routing blob.<method>() to the runtime symbol — mirroring how the arrayBuffer / clone / headers methods are routed for Response.

Repro

```ts
const res = await fetch("https://example.com/some.bin\");
const blob = await res.blob();
console.log("size:", blob.size, "type:", blob.type); // correct
const buf = await blob.arrayBuffer(); // currently undefined / no-op
const bytes = new Uint8Array(buf);
console.log("actual length:", bytes.length); // 0
```

Workaround in the meantime

Skip `blob()` entirely and call `response.arrayBuffer()` directly — that path now returns real bytes (#232).

Pointers

• Code site: `crates/perry-stdlib/src/fetch.rs::js_response_blob` — currently carries a `TODO(#NNN follow-up)` comment that fix(fetch): copy body bytes into response.arrayBuffer() result #232 will be updated to reference this issue's number once filed.
• Reference shape for the dispatch wiring: `crates/perry-codegen/src/lower_call.rs:4720-4729` (the `arrayBuffer` / `blob` arms in the response-method dispatch table).
• Reference shape for the runtime FFI: `crates/perry-stdlib/src/fetch.rs::js_response_array_buffer` (post-fix(fetch): copy body bytes into response.arrayBuffer() result #232).