Reject mismatched executor input and output lengths

[BJMCox]

Fixes #550.

Validate equal lengths before serial or threaded execution. Equal-length arrays with shifted axes retain positional mapping.

Main code: +4/-4.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.45%. Comparing base (6e8795f) to head (0bd019b).

Files with missing lines	Patch %	Lines
src/utils/executors.jl	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #558   +/-   ##
=======================================
  Coverage   65.45%   65.45%           
=======================================
  Files         121      121           
  Lines        7060     7060           
=======================================
  Hits         4621     4621           
  Misses       2439     2439           

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