feat: Drop persistent-store cache after FDv2 in-memory store init

[keelerm84]

Summary

With FDv2 the in-memory store retains every flag and segment, so the persistent-store wrapper's go-cache instance becomes dead weight once setBasis flips the active store to memory. This change drops the cache at that point to reclaim the in-memory footprint and to eliminate the CacheForever leak.

• persistentDataStoreWrapper now holds its cache behind atomic.Pointer[cache.Cache]. Every method does a single Load at the top of the call and works with that snapshot, so concurrent reads never block on a mutex and the previous status-poller-vs-Store.mu race is closed by atomic semantics. DropCache is a one-shot Swap(nil) followed by Flush on the returned cache; safe to call multiple times.
• The FDv2 store coordinator captures any DropCache-capable persistent store via an interface{ DropCache() } assertion and invokes the drop in setBasis, between s.active = s.memoryStore and the persistent Init() call. Order matters: dropping first means the wrapper's Init skips the cache-rebuild loop on the cache we are about to discard. Custom subsystems.DataStore implementations that don't satisfy the interface continue to work unchanged.
• Godoc on CacheTime / CacheSeconds / CacheForever / NoCaching now notes that under FDv2 the cache only governs the bootstrap window and may be deprecated in a future major version. No public API change.
• New tests cover the wrapper's drop semantics (idempotency, write-through after drop) and the coordinator's invocation (drop on full payload, no drop on deltas, no-op for non-cache-dropping stores). All -race clean.

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Note

Medium Risk
Touches core data-store caching and initialization paths, adding atomic cache swapping and new lifecycle behavior; mistakes could cause stale reads or missed cache writes during concurrency.

Overview
Drops the persistent-store in-process cache once FDv2 switches to the in-memory store, reclaiming memory and avoiding long-lived CacheForever caches after initialization.

The persistent store wrapper now keeps its go-cache behind an atomic.Pointer and adds DropCache() (also called on Close()), updating all cache-using methods to operate on a single loaded snapshot and to tolerate concurrent cache removal.

The FDv2 Store coordinator detects persistent stores that implement DropCache() and calls it during full-basis initialization (not deltas) before persisting via Init(). Tests were added/expanded to cover cache dropping semantics and coordinator invocation, and persistent-store cache configuration docs now clarify it only affects the bootstrap window under FDv2.

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