Add Client#evaluate_flags(distinct_id, ...) returning a
FeatureFlagEvaluations snapshot, and a flags: option on capture so
a single /flags call can power both flag branching and event
enrichment per request.

The snapshot exposes is_enabled, get_flag, get_flag_payload, plus
only_accessed / only([keys]) filter helpers. flag_keys: scopes the
underlying /flags request itself. is_enabled and get_flag fire
$feature_flag_called events with full metadata (id, version, reason,
request_id), deduped through the existing per-distinct_id cache.
get_flag_payload does not record access or fire an event.

The dedup + capture in get_feature_flag_result is extracted into
_capture_feature_flag_called_if_needed and shared between the existing
path and the snapshot's access-recording.

Existing is_feature_enabled, get_feature_flag, get_feature_flag_result,
get_feature_flag_payload, and capture(send_feature_flags:) continue
to work unchanged.

Generated-By: PostHog Code
Task-Id: fe67a5bd-7d33-4568-9148-39d181660a5a

Mirrors the changes that landed on the Python PR after review:

- only_accessed returns an empty snapshot when nothing has been
  accessed (drops the warn-and-fall-back-to-all-flags behavior — that
  was a misguided safety net that surprised callers in the
  early-pre-access pattern).
- capture(flags:, send_feature_flags:) now warns when both are passed,
  uses the snapshot, and ignores send_feature_flags. The snapshot
  guarantees the event carries the values branched on; the precedence
  was previously implicit.
- $feature_flag_called events now carry response-level errors
  (errors_while_computing_flags, quota_limited) combined with per-flag
  errors (flag_missing) as a comma-joined $feature_flag_error, matching
  the granularity of the legacy single-flag path.
- capture_exception now accepts a flags: kwarg and forwards it to the
  inner capture() so $exception events can carry the same flag context
  as other events.
- Phase 2 deprecation warnings ship alongside Phase 1: is_feature_enabled,
  get_feature_flag, get_feature_flag_result, get_feature_flag_payload,
  and capture(send_feature_flags:) emit Kernel.warn(..., category:
  :deprecated) pointing at evaluate_flags(). Public methods bypass
  each other internally (via a new private _get_feature_flag_result)
  so a single user-level call emits exactly one warning.

Generated-By: PostHog Code
Task-Id: fe67a5bd-7d33-4568-9148-39d181660a5a

Snapshot:
- Rename is_enabled → enabled? for Ruby idiom (the legacy
  client-level is_feature_enabled keeps its name but is now deprecated).
- Canonicalize the recorded $feature_flag_response across
  enabled?/get_flag so a variant flag accessed both ways collapses to
  one $feature_flag_called event (with the variant value) instead of
  two with mismatched booleans. Same fix for missing flags: response
  is nil regardless of which method was called.

Client / poller:
- Drop the feature_flags_log_warnings config option in favor of
  standard log-level gating: warnings flow through logger.warn so
  callers silence them via their existing log config (mirrors the
  posthog-android feedback).
- Skip the remote /flags call when flag_keys are all locally
  resolved. Without flag_keys we still hit the server because we don't
  know about server-side-only flags.
- Forward disable_geoip to the /flags request body as geoip_disable.
- Use a Set for flag_keys lookup to keep the per-flag check O(1).
- Normalize remote and local-eval payloads via
  FeatureFlagResult.parse_payload (made public — same contract as
  FeatureFlagResult.from_value_and_payload uses internally).
- Guard capture(flags:) against non-snapshot values: log a warning
  and ignore instead of raising NoMethodError.
- Move _get_feature_flag_result into the private section.

Deprecation warnings:
- Drop the :deprecated category — Ruby suppresses those by default
  since 2.7.2 and most users wouldn't see them. Use Kernel.warn with
  a [posthog-ruby] DEPRECATION prefix; standard log-level / IO
  silencing still works.
- Emit each deprecation at most once per (method, client) pair via
  Concurrent::Set so high-volume callers don't pay a per-call cost.

Generated-By: PostHog Code
Task-Id: fe67a5bd-7d33-4568-9148-39d181660a5a

The changeset was written before the is_enabled → enabled? rename
that came out of Dustin's review; align the example snippet and
description so the generated CHANGELOG entry uses the final API
naming. Also softens "DeprecationWarning" to "one-time deprecation
warning per method" since we dropped the :deprecated category in
favor of always-visible Kernel.warn.

Generated-By: PostHog Code
Task-Id: fe67a5bd-7d33-4568-9148-39d181660a5a