runtime: new Uint8Array(arrayBuffer) does not produce a live view; second view of same ArrayBuffer reads zeros #579

Repro

const ab = new ArrayBuffer(4);
const v1 = new Uint8Array(ab);
v1[0] = 10; v1[1] = 20; v1[2] = 30; v1[3] = 40;

const v2 = new Uint8Array(ab);   // second view of the SAME buffer
console.log(v2[0], v2[1], v2[2], v2[3]);

bun main.ts → 10 20 30 40 (v2 reads what v1 wrote, since both share ab)
perry main.ts → 0 0 0 0

Per spec, every new Uint8Array(arrayBuffer) aliases the underlying ArrayBuffer — multiple views see each other's writes. In Perry, each call seems to allocate its own backing storage and forget the ArrayBuffer.

Why this matters

Discovered while running @bradenmacdonald/s3-lite-client post #575 + #576 fix (#551). The package's sha256hmac helper does:

const signature = await crypto.subtle.sign("HMAC", keyObject, data);
return new Uint8Array(signature);   // signature is an ArrayBuffer per Web Crypto spec

The returned bytes are then hex-encoded for the AWS signature. Since the second view doesn't see the original buffer's contents, the hex signature is empty/wrong — every AWS-signed URL fails authentication.

Beyond s3-lite, this breaks every Web Crypto consumer: AWS SigV4, JWT (HS256/RS256), OAuth, web push (VAPID), Google Cloud signed URLs. The Web Crypto contract returns ArrayBuffer and consumers wrap with new Uint8Array(buf) — that's the standard idiom and it has to work.

Also breaks fetch body handling, file I/O via Blob, anywhere data crosses an ArrayBuffer boundary.

Suggested implementation

new Uint8Array(arrayBuffer) needs to produce a view that aliases the same backing storage as arrayBuffer. Currently it appears to either copy or reset to zeros. Search crates/perry-runtime/src/typedarray.rs for the constructor — the ArrayBuffer-input arm should not allocate fresh storage; it should record a pointer + offset + length into the shared buffer.

If ArrayBuffer itself isn't currently a real heap-allocated shared object in Perry, that's the underlying gap to fix first.

Acceptance

The repro prints 10 20 30 40. Plus regressions:

• new Uint8Array(ab, byteOffset, length) — partial view
• new DataView(ab) reads/writes through the same buffer
• Uint16Array, Float32Array, etc. all alias
• crypto.subtle.sign(...) → new Uint8Array(result) produces correct HMAC bytes (verified against AWS SigV4 test vectors at https://docs.aws.amazon.com/general/latest/gr/signature-v4-examples.html)
• await response.arrayBuffer() then new Uint8Array(buf) reads body bytes

Refs #551