feat: Add the FDv2 cache initializer and per-context freshness to the client

[beekld]

Summary

Adds an FDv2 cache initializer so the client can begin evaluating from its local per-context cache before the network responds, and tracks how fresh each context's cached data is.

• A cache hit produces a full basis with no selector. The cached flags are applied so evaluation can begin, but the missing selector keeps them from becoming a delta basis, so initialization continues to a network source for verified data and a real selector. A miss produces a "none" result, and the next initializer runs.
• Cache-sourced data is marked so it is not written back to the cache it came from.
• The client records a freshness timestamp per context, keyed by a hash of the full context, updated when a payload arrives or a "none" intent confirms the current data, and persisted alongside the cached flags.

Design note: this client persists a per-context freshness timestamp keyed by a hash of the full context, matching Android. Flutter's FDv2 freshness is different -- an in-memory, per-synchronizer timestamp that is neither persisted nor keyed by context.

Internal only. Not user-selectable yet.

---

Note

Overview
Adds an FDv2 cache initializer so startup can apply locally persisted flags before the network responds. A cache hit yields a full changeset without a selector (so deltas cannot treat unverified cache as a basis); a miss yields a none intent and the initializer chain continues. Cache-sourced applies are marked from_cache and are not written back or treated as service-confirmed.

Flag persistence now exposes ReadCached / ReadFreshness, records per-context freshness (hashed full context attributes) when the service sends a payload or a none confirmation, prunes freshness with LRU limits, and refuses to report freshness after flag data is evicted or overwritten under the same canonical key. FlagManager::Cache() exposes the persistence layer for the initializer.

Tests cover cache hit/miss behavior, freshness semantics, and eviction edge cases.

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[kinyoklion]

Not directly related to this PR, but it seems like ValueAsOpt may need some hardening.

[beekld]

Not directly related to this PR, but it seems like ValueAsOpt may need some hardening.

I filed a task to follow up on this.