Reimplement: Dedup (aka DISTINCT) support on PhysicalPlan

This issue tracks reimplementation of the work from stale PR #1914, which is being closed because it is too out of date to merge directly.

    Original PR: https://github.com/clockworklabs/SpacetimeDB/pull/1914
    Original author: @mamcx
    Original branch: `mamcx/distinct`
    Base branch: `master`

    ## Original PR summary

    # Description of Changes

As the title says.

Expected complexity level and risk

1

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• Write a test you've completed here.

• Write a test you want a reviewer to do here, so they can check it off when they're satisfied.

    ## Follow-up
  
    - Reimplement this change in a fresh PR against current `master`.
    - Carry forward any still-relevant context from the original PR discussion and review.
    - Link the new implementation PR back to the original stale PR for historical context.
  

Start by reviewing stale PR #1914 and comparing its DISTINCT implementation with the current master branch, focusing on PhysicalPlan. Carry forward relevant review context, reimplement the support against current master, and add or preserve the completed test coverage so DISTINCT behavior is verified.