feat: expose Big Segments via public Client and Config APIs

[beekld]

Expose Big Segments to customer code

Makes the server-side Big Segments feature reachable from customer code: a public config method to wire a store, and a status provider on the client for querying store health and subscribing to changes. Builds on SDK-2366-2.

What's in

• Promotes BigSegmentsBuilder to the public include tree and adds ConfigBuilder::BigSegments(BigSegmentsBuilder), surfaced as an optional Config::BigSegments().
• New public IBigSegmentStoreStatusProvider and BigSegmentStoreStatus (IsAvailable()/IsStale()), plus Client::BigSegmentStoreStatus().
• ClientImpl constructs the BigSegmentStoreWrapper from config, hands it to the evaluator, and starts its background poll. An internal adapter exposes the wrapper's status broadcaster as the public provider.
• BigSegmentsBuilder::Build() now returns a new error code when the store is null.

Design notes

• The status surface mirrors DataSourceStatus: the accessor is Status(), and listeners use OnBigSegmentStoreStatusChange(handler) -> IConnection.
• When no store is configured, the provider reports the store as unavailable and registered listeners never fire.

Testing

Unit tests added for the new behavior.

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Note

Medium Risk
Changes flag evaluation wiring and adds a new public API surface; misconfigured stores now fail at config build instead of silently disabling Big Segments.

Overview
Exposes Big Segments on the public server SDK so apps can configure a store and observe store health, parallel to existing data-source status.

Configuration: ConfigBuilder::BigSegments(BigSegmentsBuilder) wires optional BigSegmentsConfig into Config; omitting it leaves Big Segments disabled. BigSegmentsBuilder::Build() now returns tl::expected and fails with kConfig_BigSegments_NullStore when the store pointer is null (previously a null store was allowed).

Client runtime: When Big Segments are configured, ClientImpl creates BigSegmentStoreWrapper, starts its metadata poll, and passes it into the evaluator; otherwise the wrapper stays null.

Status API: New BigSegmentStoreStatus (IsAvailable / IsStale) and IBigSegmentStoreStatusProvider with Status() and OnBigSegmentStoreStatusChange → IConnection, exposed as Client::BigSegmentStoreStatus(). An internal provider adapts the wrapper; with no store configured it reports unavailable / not stale and listeners are no-ops.

Unit tests cover builder validation, config round-trip, and status provider behavior (including transitions).

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