fix(cluster): snapshot mutable binary source chunks

[tomdps]

Parent

• feat(cluster): validate GraphSpec and input before remote allocation #883
• Follow-up for PR feat(cluster): validate GraphSpec and input before remote allocation #891 after merged dependent PR feat: fix(cluster): make bounded source reading runtime-portable #893
• Review: feat: fix(cluster): make bounded source reading runtime-portable #893 (comment)

Problem

The runtime-portable readBoundedSource implementation retains caller-owned Uint8Array views until iteration completes. An async source may reuse or mutate a yielded backing buffer before the reader assembles the payload, overwriting earlier bytes and producing malformed or semantically different JSON.

Scope

• Snapshot every caller-owned binary chunk before requesting the next async-iterator item. String chunks produced by the reader remain owned and need no redundant copy.
• Preserve the exact byte ceiling, strict UTF-8/JSON behavior, abort behavior, Buffer-free runtime portability, and existing ClusterRequestError contracts.
• Add a regression test whose async source reuses and mutates one backing Uint8Array across yields; assert the originally yielded bytes are decoded.
• Cover a subarray/shared-backing-buffer case so retaining a view cannot regress.
• Avoid unrelated changes and do not modify the known unrelated E2E cleanup-race tests.

Delivery

Target the existing PR #891 head branch through a dependent PR whose base is zeroshot/platinum-forest-30. Parent PR #891 remains paused until this fix lands and all required checks pass.