swift-test-bench.sh: stream build output so a long build is not stopped as a hung measurement
scripts/measure/swift-test-bench.sh (#27352) now sends its FFI and swift build output to the step log as it runs, instead of only to a file.
Its first fleet run, CmuxMobileKit's TerminalInputThroughputTests/keystrokeThroughputAt500msRTT, built the FFI xcframework in 144 s. Then it printed nothing while swift build wrote only to .build/swift-test-bench/build.log. After 45 minutes of silence the controller stopped the step as a hung measurement, which is the fleet's silence limit for measure steps. The step directory was cleaned up afterwards, so the log is gone. With this change a long build shows its progress, and a real hang shows the last line it reached in the step log.
Exit status still comes from the build command (PIPESTATUS, not the tee), and a failed build still lists its error: lines.
Performance
No perf numbers: this is a logging fix to the measurement script, with no speed change.
Validation
bash scripts/measure/tests/swift-test-bench-args.test.sh passes, and shellcheck -S warning is clean. The next fleet run of the benchmark exercises the change.
Risk
Low. The step log grows by the build's output, and nothing else changes.
Agent session: agent session
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Summary by cubic
Streams the FFI and swift build output from the measurement script to the step log as it runs, so a long build is no longer stopped as a hung measurement.
On the first fleet run the step printed nothing for 45 minutes while swift build wrote only to .build/swift-test-bench/build.log, and the controller stopped it at the measure step's silence limit. Build output now reaches the step log in real time. The exit status still comes from the build command (PIPESTATUS, not tee), and a failed build still lists its error: lines. The step log grows by the build output. The script now fails fast with exit code 3 if a build log cannot be written, instead of continuing with an incomplete log.