feat(tools): S8 zero-velocity inspector — real EuRoC stationary window → ZUPT decision

[Ravenwater]

Per-stage inspector for epic #371. Depends on #372, #373. Renderer: filter-internal tier.

• Input: IMU window + state (trace).
• Transformation: the zero-velocity detector + ZUPT update.
• Output: ZUPT decision + (if applied) updated state/covariance.
• Viz: detector statistic vs threshold over time, covariance effect of an applied ZUPT.