Allow type override in := when i is an ordering call

[venom1204]

closes #2925

Identifies order() or forder() in i as a full-column permutation and permits a column type change (plonk) during assignment. Previously, any non-missing i was treated as a subset update, which strictly forbade type overrides.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (25c9dde) to head (21caf0d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7879   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17305    17316   +11     
=======================================
+ Hits        17135    17146   +11     
  Misses        170      170           

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[github-actions]

• HEAD=subset much slower for transform improved in #5493
  

Generated via commit 21caf0d

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	6 minutes and 33 seconds
Installing different package versions	13 minutes and 6 seconds
Running and plotting the test cases	5 minutes and 36 seconds