chore: Update yard tags, and locations for private implementations

[jsonbailey]

Tracked internally: SDK-1679

Note

Reorganizes private internals under LaunchDarkly::Impl, replacing global constants/utilities and updating code/tests to use the new paths and @api private tags.

• Internal structure:
  • Moved polling, streaming, requestor, LRU cache, thread pool, expiring cache, memoized value, and util to ldclient-rb/impl/....
  • Split data store kinds into Impl::DataStore with FEATURES/SEGMENTS and data_store/data_kind.rb.
• API/use-site updates:
  • Switched references from LaunchDarkly::FEATURES/SEGMENTS and Util to Impl::DataStore::* and Impl::Util across SDK.
  • Config.default_cache_store now uses Impl::ThreadSafeMemoryStore; added impl/cache_store.
  • Top-level requires trimmed to public APIs in lib/ldclient-rb.rb.
• Docs:
  • Converted @private to @api private where applicable.
• Tests:
  • Updated specs to new modules/constants and paths; adjusted expectations accordingly.

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