Rerank a list of items through an optional 3-stage pipeline: sort → diversity → limits_by_field.

Each item must have item_id; other fields are used by the pipeline stages. The final order is returned with position-based scores in [0, 1] (first item = 1.0, last = 0.0 for n > 1).

Sort Stage (optional):

• sort: Multi-field sort. params.fields (array of field names), params.direction (array of asc or desc, same length as fields).
• linear: Weighted sum. params is a list of { field, weight }. Items are scored by sum(field_i * weight_i) and ordered descending.
• mix: Interleave by percentage. params is a list of { field, percentage, direction }. Percentages are normalized; items are taken in round-robin from each sorted list.

Diversity Stage (optional):

• fields: Round-robin over groups. params.fields = list of field names. Items are grouped by field values and interleaved.
• semantic: MMR-style diversity. params.lambda in [0, 1] (trade-off between relevance and diversity), params.horizon >= 5 (window size). Items should have an embed field (numeric array).

Limits-by-Field Stage (optional):

• params.every_n: Slot size (>= 2). params.rules: list of { field, limit }. Within each slot of size every_n, at most limit items per distinct value of field are allowed.