feat: Add C bindings for Big Segments

[beekld]

Summary

Exposes the Big Segments configuration and store-status surface in the C API. Adds:

• LDServerBigSegmentsBuilder with ContextCacheSize, ContextCacheTimeMs, StatusPollIntervalMs, and StaleAfterMs setters.
• LDServerBigSegmentStorePtr -- opaque store handle produced by integration libraries (e.g. the coming Redis and DynamoDB Big Segments sources) and consumed by LDServerBigSegmentsBuilder_New.
• LDServerConfigBuilder_BigSegments, which attaches a Big Segments builder to the SDK config.
• LDServerBigSegmentStoreStatus opaque handle with _Available and _Stale getters.
• LDServerBigSegmentStoreStatusListener struct + _Init, LDServerSDK_BigSegmentStoreStatus_OnStatusChange, and LDServerSDK_BigSegmentStoreStatus_Status for status queries and change notifications.

Design decisions worth flagging:

• The _Available / _Stale getters are bare predicates, not _IsAvailable / _IsStale. The C++ side uses IsAvailable() / IsStale(), but every existing C-binding bool-query in this repo drops that prefix -- LDServerSDK_Initialized, LDContext_Valid, and so on. The new C names follow that pattern.
• ServerBigSegmentStoreStatusCallbackFn is the one exception to the LD / LDServer prefix rule. It mirrors the existing ServerDataSourceStatusCallbackFn typedef (sdk.h:863), which drops the prefix. New code matches that quirk to stay consistent within the same header.

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Note

Low Risk
Additive public C API and binding glue only; no changes to evaluation or store logic beyond wiring existing C++ types. Main integration risk is correct store pointer lifetime when passed from future Redis/DynamoDB C integrations.

Overview
Adds C API surface for Big Segments on the server SDK, parallel to existing C++ BigSegmentsBuilder and data-source status patterns.

Configuration: New LDServerBigSegmentsBuilder wraps an integration-provided LDServerBigSegmentStorePtr (ownership transferred to the SDK) with tunables for context cache size/TTL, status poll interval, and stale-after threshold. LDServerConfigBuilder_BigSegments consumes the builder into the config, matching other consumed builders (FDv2, lazy load).

Runtime status: LDServerBigSegmentStoreStatus exposes _Available / _Stale predicates; LDServerSDK_BigSegmentStoreStatus_Status and listener APIs (LDServerBigSegmentStoreStatusListener, OnStatusChange) mirror the existing data-source status C bindings.

Implementation is thin C shims in big_segments_builder.cpp, builder.cpp, and sdk.cpp, plus binding tests for config attachment, default status when no store is configured, and listener registration.

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