Harden TimePredictor pace_factor: positive-pace guard and clamp

[moveson]

Background

Split out of #2169 (implemented in #2237) during plan review. #2237 gates the statistics pool on use_for_projections and makes a degenerate limits band evaluate as "unknown", but two pace-factor weaknesses in TimePredictor (app/services/time_predictor.rb) remain:

1. Latent crash: zero typical completed time

measurable_pace? checks truthiness only:

def measurable_pace?
 completed_lap_split.distance_from_start.positive? && actual_completed_time && typical_completed_time
end

typical_completed_time == 0.0 is truthy, so pace_factor = actual_completed_time / typical_completed_time is float division by zero → Infinity, and limits' (limit * pace_factor).to_i raises FloatDomainError (0 * Infinity → NaN does too). Fix: require actual_completed_time&.positive? && typical_completed_time&.positive?, falling back to the existing unmeasurable-pace convention of 1.

2. Unbounded pace factor

In the #2169 incident, a poisoned pool produced pace_factor ≈ 1e-4, collapsing the limits band to zero width so every real time flagged "bad". The pool gating in #2237 makes this far less likely but not impossible (bad data in flag-true events — mis-keyed year, timezone error). Clamping bounds the blast radius:

MIN_PACE_FACTOR = 0.1
MAX_PACE_FACTOR = 10.0

(actual_completed_time / typical_completed_time).clamp(MIN_PACE_FACTOR, MAX_PACE_FACTOR)

Legitimate pace factors sit roughly in 0.3–3, so [0.1, 10] never binds on real data; clamp-then-use beats treat-as-unmeasurable because an extreme pace signal still carries direction, just not magnitude.

Suggested specs

Build a predictor with calc_model: :stats and stub SplitTimeQuery.typical_segment_time (arity-preserving idiom already used in segment_time_calculator_spec):

• huge average (anachronistic pool): data_status(sane_seconds) is not "bad"; pace_factor == 0.1
• average: 0.0: no FloatDomainError; status nil; pace_factor == 1
• negative average: status nil, pace_factor == 1

Relates to #2169

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