docs: finalize RFC 0005 — arbitrary string row ids (accept)

[hevbot]

Links issue #20. This intentionally does not close it; implementation follow-up issues can be filed after merge.

Rendered links:

• Branch view: https://github.com/hev/search/blob/docs/rfc-0005-finalize/docs/rfcs/0005-arbitrary-row-ids.md
• Rich diff view: https://github.com/hev/search/pull/21/files#diff-2ab8b1973f0f8f5b61b713fd40037ba12782893c3a92b9c26a3ce0f3edd98cb3

Resolved open questions:

• Enum vs branch: resolved to RowId / RowIdType enums end to end. Evidence: crates/hevsearch-core/src/result.rs.
• Schema choice: resolved to per-namespace UInt64 or Utf8 in schema_for_kind. Evidence: crates/hevsearch-core/src/manager.rs.
• Mixed-type writes: resolved to fixed namespace id_type; wrong-type upsert/import/delete rejects. Evidence: upsert, import_arrow_with_distance_metric, delete_ids.
• Cursor re-encode: resolved to versioned v1:u / v1:s tokens with legacy numeric decode. Evidence: encode_list_cursor, decode_list_cursor, cursor unit tests.
• Import: resolved to Arrow id as UInt64 or Utf8, checked against existing namespaces. Evidence: validate_arrow_import_schema.
• Delete quoting: resolved through RowId::to_sql_literal before id IN (...). Evidence: string delete quote regression tests.
• Multivector + string id: resolved as independent of vector kind. Evidence: validate_arrow_import_schema test with id: Utf8 plus vectors.

Open question for review:

• String id length bound: current code does not appear to enforce one. The RFC flags whether to accept unbounded Utf8 ids or file a follow-up implementation issue for a concrete byte limit.

Validation:

• Docs-only change: docs/rfcs/0005-arbitrary-row-ids.md is the only touched file.
• git diff --check passes.
• No cargo suite run; this is docs-only, and the protected fmt/clippy/test lane will still run on the PR.

[hevbot]

Factory verified: diff touches only docs/rfcs/0005-arbitrary-row-ids.md; rendered links in body; open questions explicitly flagged for review. Branched from post-#15 green main — required check should pass on its own. Ready for hev's docs review; merge = RFC accepted + factory files implementation issues.

[hev]

@hevbot deal with the merge conflict here

[hevbot]

No actual merge conflict — GitHub was showing the out-of-date-with-base banner (state was MERGEABLE/BEHIND). Branch updated onto post-#18 main; CI re-running; merge when green.