feat: Add the FDv2 data source orchestrator to the client SDK

[beekld]

Summary

Adds the client SDK's FDv2 data source -- the orchestrator that runs initializers to load a basis, then rotates synchronizers to keep it current.

Implemented here:

• Runs initializers to a basis, then rotates synchronizers on fallback and recovery conditions.
• Reports data-source status, records the environment id, and tags applied data as cache- or network-sourced.

Also moves the shared orchestration primitives out of the server SDK into libs/internal so both SDKs can share them. That server change is a behavior-neutral relocation.

Notes:

• Start() blocks on cache initialization, so cached flags are immediately evaluable. This matches the behavior of the FDv1 client. It also matches Android. However, Flutter loads the cache asynchronously instead.
• FDv1 fallback directive handling is out of scope.

---

Note

Overview
Introduces FDv2DataSource for the client SDK: it chains initializers (including synchronous cache loads before Start() returns), then synchronizers with fallback/recovery conditions, applies changesets to the flag store, publishes data-source status, tracks environment id, and marks cache vs network applies. Shutdown is guarded so late results cannot overwrite a replacement source after identify.

Shared FDv2 orchestration (Conditions, SourceManager, condition interfaces) moves from the server SDK into libs/internal, under launchdarkly::internal::data_sources. Conditions now take a SourceSignal enum instead of full synchronizer results; SourceManager is a header-only template usable by both SDKs. The server FDv2DataSystem is updated to consume the shared headers (behavior-neutral relocation aside from the signal mapping).

Adds broad unit tests for the client orchestrator and adjusts internal/server tests for the new namespaces and APIs. .gitignore ignores .env and .claude.

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[beekld]

This PR needs some more work on thread safety. Because the store can outlive the data source, the mechanism that worked in the server code isn't 100% sufficient for client code. I'll comment again when it's ready.

[beekld]

Okay, I think I fixed the fence issues by adding a future that gets taken and not resolved until any outstanding writes finish.

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