feat: Hand over the data store to the new client on re-anchor

[keelerm84]

Summary

First slice carved out of #720 (the re-anchor mechanism, SDK-2542), split for reviewability. This is the store-handover piece and is independent of the credential and relayenv changes -- it adds a latent capability that the synchronous re-anchor will use in a later PR.

SSERelayDataStoreAdapter.Build now reuses its existing wrapper when one is already present, so a concurrent-keys re-anchor can hand the populated, initialized store to the new anchor's client instead of building a fresh, empty one -- no empty-store window, no re-sync. The wrapper is refcounted: acquire() bumps the holder count on each Build reuse and Close() decrements, tearing down the underlying store only on the final release. This keeps a retiring client's Close() from pulling the store out from under the new anchor. A fully-closed wrapper refuses re-acquisition, so Build rebuilds rather than resurrecting a dead store.

Tests

• Inverts the H1/H5 re-anchor PoC tests from asserting the pre-fix (store wiped) behavior to asserting handover. H6/H7 are intentionally left as-is; they invert with the synchronous re-anchor change in a later slice.
• Adds TestRealClient_HandoverPreservesUnderlyingStore -- a real-client spike verifying ld.LDClient.Close() does not tear down the shared underlying store.
• Adds store_rebuild_after_close_test.go -- a refcount-contract regression covering the "fully-closed wrapper refuses re-acquisition" path.

Split sequence

This is PR 1 of a 4-slice split of #720: store handover (this PR) -> relayenv refactors -> credential rotator deferred-flip API -> synchronous re-anchor core.

---

Note

Medium Risk
Changes core data-store lifecycle on SDK client swap (refcounted Close and Build reuse); impact is limited to the store adapter layer in this slice, with broad regression and race tests, but mistakes could cause use-after-close or double-close on persistent stores.

Overview
Adds store handover so a concurrent-keys re-anchor can stand up a new upstream LDClient without wiping the in-memory feature store.

SSERelayDataStoreAdapter.Build now returns the existing streamUpdatesStoreWrapper when one is already parked, using acquire() to bump a holder refcount instead of building a fresh empty wrapper. Close() on the wrapper only tears down the underlying store on the final release, so a retiring client’s shutdown does not close the store while the new anchor still holds it. Fully closed wrappers refuse re-acquisition so a later Build creates a new wrapper rather than resurrecting a dead store.

Re-anchor PoC tests H1/H5 are flipped to expect handover (same store instance, data preserved through env_context). New coverage includes real-LDClient handover, rebuild-after-full-close, and refcount idempotency / handover-vs-Close race tests (intended for -race).

^{Reviewed by Cursor Bugbot for commit e598fa9. Bugbot is set up for automated code reviews on this repo. Configure here.}