feat(flags): support group-targeted and mixed-targeting flags in local eval

[patricio-posthog]

Motivation and Context

Two related gaps in the Rust SDK's local evaluation, addressed together because supporting mixed targeting requires the underlying group-flag plumbing:

1. Pure group flags (where aggregation_group_type_index is set at the flag level) weren't evaluated locally. The matcher only knew about person properties and bucketed on distinct_id unconditionally; the public Client::get_feature_flag accepted groups / group_properties but silently dropped them on the local-eval path.
2. Mixed-targeting flags (the feature-flag-mixed-targeting beta), where individual conditions set their own aggregation_group_type_index, couldn't be evaluated locally either. This breaks customers using flags in environments that can't make HTTP fallback calls (background workers, etc).

Brings posthog-rs to parity with the other PostHog SDKs (Python, Node, Go, Ruby, PHP, .NET) which already support both. Equivalent of posthog-python#523 plus the prerequisite group-flag baseline.

How did you test it?

All 112 existing tests pass. Added 7 new tests in tests/test_local_evaluation.rs:

• test_pure_group_flag_matches_with_group_passed_in
• test_pure_group_flag_returns_false_when_groups_not_passed
• test_mixed_flag_person_condition_matches_when_no_groups
• test_mixed_flag_group_condition_matches_with_group_props
• test_mixed_flag_no_match_when_both_fail
• test_mixed_flag_only_group_condition_no_groups_returns_false
• test_group_condition_uses_group_key_for_bucketing — partial-rollout test with pre-computed hashes that would catch a regression to bucketing on distinct_id

cargo fmt --check clean, cargo clippy --all-targets clean.

Breaking change at the public API

This is a minor semver bump (the SDK is at 0.5.x, pre-1.0). Public functions whose signatures changed:

• LocalEvaluator::evaluate_flag(key, distinct_id, person_properties) — now takes groups: &HashMap<String, String> and group_properties: &HashMap<String, HashMap<String, serde_json::Value>> as additional parameters.
• LocalEvaluator::evaluate_flag_simple(...) — same change.
• LocalEvaluator::evaluate_all_flags(distinct_id, person_properties) — same change.
• match_feature_flag(flag, distinct_id, properties) — now takes groups, group_properties, and group_type_mapping as additional parameters.
• match_feature_flag_with_context(flag, distinct_id, properties, ctx) — drops the distinct_id parameter (now read from ctx.distinct_id); EvaluationContext gains groups, group_properties, and group_type_mapping fields.
• Client::evaluate_feature_flag_locally(...) (both async and blocking) — adds groups and group_properties parameters.

For callers using person flags only, pass &HashMap::new() for the new group parameters.

Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog. (intentionally a minor breaking change — see above)

If releasing new changes

• Ran sampo add to generate a changeset file
• Added the release label to the PR