Refactor schema: collapse category-specific slots into canonical slot…

[diatomsRcool]

…s with modifiers

Apply category collapse design principle: when multiple variables represent the same quantitative construct differing only by a categorical qualifier, use ONE canonical slot with modifier axes instead of separate slots.

Changes:

• Add SpO2ThresholdEnum and CO2ThresholdEnum modifier axes
• Add spo2_threshold and co2_threshold modifier slots
• Replace body position slots (supine_duration_min, etc.) with body_position_duration_min + body_position modifier
• Replace apnea type slots (central_apnea_index, etc.) with apnea_index + apnea_type modifier
• Replace SpO2 threshold slots (time_below_90_pct_min, etc.) with time_below_spo2_threshold_min + spo2_threshold modifier
• Replace CO2 threshold slots (time_co2_above_50_pct, etc.) with time_above_co2_threshold_percent + co2_threshold modifier
• Update BodyPositionMetric, OxygenSaturationMetric, CapnographyMetric classes
• Document design principle in schema description

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