Chardata plus encoded datasets

[pp-mo]

Closes #6309 + various

Successor to #6850
now incorporating #6851

+ now integrated usage with netcdf load+save, to use encoded datasets

[pp-mo]

Update

merged from main to unblock CI testing

[pp-mo]

Status Update 2026-03-06

See #6919 (comment)

[scitools-ci]

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[codecov]

Codecov Report

❌ Patch coverage is 94.70199% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (043b0bc) to head (4996be2).

Files with missing lines	Patch %	Lines
...ib/iris/fileformats/netcdf/_bytecoding_datasets.py	93.92%	7 Missing and 4 partials ⚠️
lib/iris/fileformats/netcdf/_thread_safe_nc.py	71.42%	4 Missing ⚠️
lib/iris/fileformats/netcdf/saver.py	98.76%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6898      +/-   ##
==========================================
+ Coverage   90.11%   90.19%   +0.08%     
==========================================
  Files          91       92       +1     
  Lines       24912    25069     +157     
  Branches     4675     4686      +11     
==========================================
+ Hits        22449    22611     +162     
  Misses       1684     1684              
+ Partials      779      774       -5     

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[pp-mo]

Finally got all tests passing.
But in the process, realised that until at least #6920 is completed, we still can't really be confident that this is all correct.
(i.e. we know that significant additional testing is required)

So now, retargetted to feature branch + replaced this PR with #6975