Fail closed on corrupted KV reads

[johnny-emp]

Summary

• route production KV read paths through fallible try_* APIs so value-store and segment corruption return structured errors
• remove the non-PK foreign-key runtime scan fallback and fail with an integrity error when unique index state is missing
• add regression coverage for spilled value hash mismatch, KV segment corruption, secure scan fail-closed behavior, and missing FK runtime index state

Tests

• cargo check
• cargo test --lib corruption_fails_closed -- --nocapture
• cargo test --lib non_pk_foreign_key_fails_closed_when_runtime_unique_index_missing -- --nocapture
• cargo test --test read_assertions
• cargo test --test kv_updates_preflight
• cargo test --test query_integration