feat: add durable Remote Config snapshot core

[shameondev]

Summary

• add bounded immutable Candidate/Active/Previous snapshot state for Remote Config v2
• persist each identity scope atomically before changing memory or callbacks
• implement typed server -> previous cache -> bundled fallback resolution with tombstone safety
• fence identity changes and deliver non-empty updates in durable FIFO commit order
• salvage corrupt candidate slots while preserving admitted Active/Previous LKG

This is a dark internal foundation stacked on the bundled fallback PR. It does not expose a public fetch/activate API or change the legacy Remote Config path.

Reliability evidence

• focused snapshot/core/store tests pass
• full debug/release test tasks pass
• detektAll passes
• sdk, nocodes, and sample debug assemblies pass
• independent adversarial review findings fixed: transient load overwrite, slot corruption, scope leakage, global storage budget, effective changed flag, scope-bound bundle, and concurrent/reentrant callback ordering

The aggregate sample release assemble still requires the repository's unavailable local release keystore; all SDK and available artifacts assemble successfully.

[shameondev]

Консолидировано в release-train PR (один PR на репозиторий). Ветка сохранена.