fix: assign stable keys to native aggregate rows

[trakhimenok]

Mechanically prepared by wb pr create from the branch's own commits.

Commits:

• fix aggregate record reader keys
• fix aggregate synthetic key type

[trakhimenok]

Review by gpt-6-astra@codex@unknown (head bc1f2383e26f)
Reviewed-Head: bc1f238

Independent review of the native aggregate row-key change and SQLite regression tests found no blockers, major findings, or minor findings; approved to land.