Verify distributed decimal behavior on real DM MPP

[skhe]

Summary

• Add a repeatable real DM8 MPP regression covering global and local logins on two endpoints, hash-distributed DECIMAL(30,8) data, and exact batch-write round trips.
• Record the local ARM two-node MPP setup and results; update API and support docs to reflect the tested scope.

Verification

• Local Orb two-node DM8 MPP with ARM macOS Python 3.10: 12 exact rows visible globally from each endpoint; 6 disjoint local rows on each EP; union equals global rows.
• Python 3.9 syntax compilation and git diff --check passed.

The dedicated MPP regression is local because GitHub-hosted runners cannot reach this Orb cluster. The existing CI matrix remains a single-node real DM8 gate.