[Hard] Rewrite vector index for Federation & Cross-Provider

[keepsloading]

Implementation issue mapping to the Federation & Cross-Provider epic for the Protocol repository.

[keepsloading]

Friendly reminder for SSoC26 contributors: When you submit a Pull Request for this issue, please remember to also raise a dummy PR in the Memact/Context repository that references your original PR. This is required for your contribution to be considered for points on the SSoC leaderboard!

[kasim672]

@keepsloading assign me please