chore: Cleanup shuffle_writer module

[EmilyMatt]

Which issue does this PR close?

Closes #3354 .

Rationale for this change

Desperately needs a cleanup.

What changes are included in this PR?

No logic changes, I believe.
This is just...shuffling code around 👉 😎 👉
Perhaps reduced visibility of some structs for isolation and a rename or two.

How are these changes tested?

Since there's no logical changes, just making sure all the current tests pass should be enough

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.00%. Comparing base (f09f8af) to head (82561ef).
⚠️ Report is 916 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3355      +/-   ##
============================================
+ Coverage     56.12%   60.00%   +3.87%     
- Complexity      976     1475     +499     
============================================
  Files           119      175      +56     
  Lines         11743    16165    +4422     
  Branches       2251     2681     +430     
============================================
+ Hits           6591     9699    +3108     
- Misses         4012     5114    +1102     
- Partials       1140     1352     +212     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[EmilyMatt]

@andygrove Ready on my end, and looks like checks pass and all is well in the world :)

[andygrove]

Hi @EmilyMatt. I agree that the code could benefit from some cleanup, but is it possible to break this down into some smaller PRs? Perhaps start with one to introduce the new trait? This is a large PR to review, and it lacks a description of the specific changes, so I'm not sure where to start.

[EmilyMatt]

Hi @EmilyMatt. I agree that the code could benefit from some cleanup, but is it possible to break this down into some smaller PRs? Perhaps start with one to introduce the new trait? This is a large PR to review, and it lacks a description of the specific changes, so I'm not sure where to start.

Sorry, jumped far ahead of myself, will open smaller PRs^

[EmilyMatt]

First one is #3373

[EmilyMatt]

Second PR: #3374
(Documenting on this PR for posterity)

[EmilyMatt]

3/4 PR:
#3385