A collection GET whose `@computed` getter throws mid-stream returns **HTTP 200 with `Error.toString()`/a stack t

From dispatch finding (task `qa-wave-2026072920`):

- F-258 — A collection GET whose `@computed` getter throws mid-stream returns **HTTP 200 with `Error.toString()`/a stack trace as the response body**: the status is flushed before the getter runs, so the client sees success plus unparseable JSON, and one poisoned record corrupts the entire response. The single-record GET of that same record degrades correctly (clean 500 + JSON error body), which localizes this to the streaming-collection path. Same 200-wrapping-an-error family as F-186.2.

Start in the streaming-collection GET path and compare it with the single-record GET, which already returns a clean 500 and JSON error body. Reproduce the issue with a collection containing a record whose @computed getter throws; done means the collection no longer returns HTTP 200 with a stack trace or unparseable body.

Written by the indexing model from the issue text.