Pipeline workspace artifact exports

[SaladDay]

Completed Turns wait for output Artifact capture, whose sequential File reads amplify remote latency. Export files in batches of at most four and the declared handle limit, with one ordered tar writer. Preserve file size/mtime validation, symlink handling, all export bounds, and descriptor identity; cancel and join the batch on errors and settle handles before releasing owner references.

A seven-file real File fixture preserves byte-identical archives and all 41 RPCs, while 20/100 ms injected handler latency falls about 36.6%. Zero-delay samples show no reliable improvement; cumulative Go allocations increase by roughly 80 KiB. Readers hold at most one bounded File response each and share a 32 KiB copy buffer per batch, without whole-file buffering. Measurements precede the final error-cleanup cancellation follow-up; successful scheduling is unchanged. Live latency is not yet requalified.

Validation: focused agenthost/dispatch tests, relevant race tests ×20, exact 200 MiB streaming and advertised handle limits 1–4, failure/cancellation/late-acquisition cleanup, vet, check-names, check-docs and check-ci. Independent full-diff review found one cancellation issue; corrected with a three-line change and an asymmetric blocked-read regression (old fails, correction passes race ×20). No other substantive findings.

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