Guard the TimePredictor pace factor against non-positive baselines

[moveson]

Summary

Implements the surviving piece of #2238 after design review (the clamp was considered and deliberately dropped — see below). Written TDD-style: the failing specs were committed first, then the fix.

measurable_pace? checked truthiness only, and 0.0 is truthy, so:

• A zero typical completed time produced Infinity via float division, and limits' (limit * pace_factor).to_i raised FloatDomainError — an unrescued exception in SetEffortStatus, which runs in the live-entry submission path. Reproduced verbatim in the first commit's spec run.
• A negative actual completed time (times imported before the effort's start) produced a negative pace and therefore a negative limits band, silently flagging every subsequent healthy time "bad" with nonsense reasons — pointing reviewers away from the actually-broken time.

measurable_pace? now requires positive actual and typical completed times, falling back to the existing unmeasurable-pace convention of 1, so per-segment checks flag the genuinely broken times instead.

Why no clamp

A [0.1, 10] pace-factor clamp was implemented and then removed during review. Rationale: with #2237's pool gating in place, a corrupt flag-true baseline is rare — and when it happens, the un-clamped failure mode (mass bad flags) is loud and self-announcing, which is exactly how the #2169 incident was discovered. Clamping without an observability channel would convert that rare loud failure into a rare silent one, asserting "good" precisely when the evidence is least trustworthy. Scout capture on clamp engagement was also considered and rejected as alert-fatigue noise.

Resolves#2238

Testing

• New #data_status with a degenerate stats baseline specs (stubbed per-segment pool averages): zero completed-segment average → no raise, evaluates against the unscaled band (pre-fix: FloatDomainError: Infinity); negative average → same (pre-fix: everything "bad" via a negative band).
• All 20 time predictor examples pass; wider suites green (interactors, intended time calculator, enrich/verify raw time row, segment container/calculator). rubocop clean.

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