Gradual rollout evaluation for feature flags.

A rollout definition decides whether a specific entity (user, tenant, session…) gets the feature. Three strategies are supported:

• %{type: :percentage, value: 25} — the entity falls into the first 25% of a deterministic hash bucket. Same entity → same answer across restarts.
• %{type: :user_list, users: ["lorenzo"]} — explicit allow-list.
• %{type: :attribute, key: "tenant", values: ["acme"]} — the context attribute key must be one of values.

Legacy integer rollouts (rollout: 25) are treated as a percentage.

Context

The evaluation context is a map with string keys, typically the request context of the caller:

%{"user" => "user_42", "tenant" => "acme", "session" => "s-1"}

Determinism

Percentage bucketing uses :erlang.phash2/2 with a 100-bucket range, keyed by {flag_name, entity} so the same entity always lands in the same bucket, independent of process restarts or VM restarts.