Vectorised evaluation

[khvn26]

Together with Flagsmith/flagsmith#7419, this unlocks:

• read-latency SLO tightening for 100M+ environments
• backfill SLO bucket tightening for 10M+ environments

Scope

• Public evaluate_conditions(context, conditions) -> list[bool] primitive in flagsmith-engine.
• Vectorised evaluate_conditions_batch(context_columns, conditions) using Polars or Arrow.
• services.evaluate_atom in Flagsmith Core swaps in the new primitive.

Acceptance criteria

• The batch primitive achieves ≥ 100× speedup vs context_matches_condition over the same operator mix on 1M identities.
• Swap-in does not change backfill or maintenance code beyond services.evaluate_atom.