Implement arbitrary string row ids

[hevbot]

Summary

• add namespace-aware Arrow import preflight so existing-namespace id_type/kind/dim/metric mismatches return 400 before a background import is accepted
• add API coverage for string id upsert -> query -> list pagination -> facet -> namespace info and wrong-type rejection
• add API import coverage for id: Utf8 and string-import mismatch against a fixed u64 namespace

Closes #31

Test Evidence

MinIO isolation: dedicated container hevsearch-rowids-minio on HEVSEARCH_S3_ENDPOINT=http://127.0.0.1:19001; bucket hevsearch-test.

Passed:

• cargo test -p hevsearch-core --test manager_local_fs string_ids -- --nocapture
• cargo test -p hevsearch-api --test api_string_ids -- --ignored --nocapture
• cargo test -p hevsearch-api --test api_import import_accepts_utf8_id_stream_and_rejects_existing_u64_namespace -- --ignored --nocapture
• cargo fmt --all --check
• git diff --check
• cargo clippy --workspace --all-targets -- -D warnings

Broad suite attempted:

• cargo test --workspace -- --ignored --skip _aws --skip _100_runs_ failed during linking, before tests ran: ld terminated with signal 9 [Killed] while linking manager_upsert_query, consistent with memory pressure from broad parallel test linking.
• Retried as cargo test --workspace -j1 -- --ignored --skip _aws --skip _100_runs_; it produced no output for several minutes after the linker retry and was interrupted.

[hevbot]

Factory verified: RFC 0005 implemented per accepted spec, 4-file scoped diff, own-MinIO test isolation honored, required check green. Merge-ready — this is the demo-family conversion unblock.