One record backend over EdgeZero KvStore; delete the Fastly ec_kv.rs #1587

Summary

One record backend over EdgeZero KvStore; delete the Fastly ec_kv.rs; adapters without a supported store reject identity configuration at startup, with a test.

Part of #1558, server-side identity on every adapter. Plan ID T6, planned in wave 6 (MVP: Fastly and Axum); priority P1, size L. Design: spec; order and dependencies: plan.

Requirements

ID-2 One shared record store

The identity logic stays in core and becomes awaitable (#1526). It runs on one EcKvStore over EdgeZero's KvStore (DEC-3). A store that cannot meet the contract makes startup reject identity configuration with a clear error.

Acceptance criteria

• ID-2: No adapter crate's non-test code implements EcKvStore, and a store reporting KvError::Unsupported for the contract fails startup with identity enabled

Dependencies

Blocked by:

• Integration-branch tag with the record contract: EZI-2a and EZI-2b edgezero#489 Integration-branch tag with the record contract: EZI-2a and EZI-2b (new in this plan)
• Make shared EC persistence awaitable #1522 Make shared EC persistence awaitable

Blocks:

• Identity on Axum through T21's shared app #1574 Identity on Axum through T21's shared app (new in this plan)
• Identity on Cloudflare through T21's shared app with EZI-2c's store, passing T5's suite #1588 Identity on Cloudflare through T21's shared app with EZI-2c's store, passing T5's suite (new in this plan)