#433 Completed-Rust-check fixture setup no longer races its capture budget

Accepted agent delivery; branch-keyed search descriptor included.

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Summary by cubic

Stops the completed-Rust-check fixture from racing its capture budget by letting tests set a longer deadline than the production budget.

• Adds begin_until for specifying a custom deadline and uses a 60-second fixture budget in the unix test.
• Adds a land-rust-check-fixture-budget harness descriptor pointing at harness-goldens.

Confidence Score: 5/5

This PR appears safe to merge.

Summary

This PR gives shared completed-Rust-check fixtures more time to capture and save their inputs.

• Rust-check test fixtures get a longer window to save completed checks.

_{Reviews (1) · Last reviewed commit: "landing: describe rust-check-fixture-bud..." · Reviewed by Greptile}