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.