Stabilize short Gate 6N build measurements

[hiroki23]

Summary

• batch eight direct application-build launches per Gate 6N elapsed observation
• retain raw batch duration and a per-launch normalized duration
• keep independent single-launch RSS, eleven retained observations, the 25% comparison, and the 2x catastrophic threshold unchanged
• document the measurement boundary and cover normalized repeated measurements

Verification

• python3 -m unittest tools/gate6n-measure-test.py
• dash -n tools/gate6n-linux-amd64.sh
• bash --posix -n tools/gate6n-linux-amd64.sh
• trb fmt --check .
• git diff --check

Closes #206