fix: khive-retrieval feature-gated test failures (persist, storage-adapters)

[ohdearquant]

Problem

Two optional features have test failures:

1. persist feature — 36 compilation errors

• Missing use khive_hnsw::NodeId; in persist/tests.rs
• khive_db::SqliteStore referenced but doesn't exist in current API
• Files: src/persist/tests.rs:47, src/replay/engine_replay.rs:847, src/weights/engine_weights.rs:301

2. storage-adapters feature — 5 runtime panics

• Tests require sqlite-vec extension loaded at runtime
• StorageBackend::memory() doesn't load it
• Tests should be #[ignore] with comment explaining runtime dependency
• File: src/adapters/mod.rs:203

Impact

Default feature set (148 tests) is unaffected. These only matter when features are explicitly enabled.

Fix

1. Add missing imports for persist
2. Either mock VectorStore or #[ignore] tests that need sqlite-vec

Source

Critic review of PR #302.