fix: Serialize index of control-flow ports

[acl-cqc]

and remove logic duplicated in OpType::port_kind

The main goal here is to enable hugr-py to read in CFGs serialized from Rust, by changing the Rust to output in the same form as the python.

So, to test this...I should write out a file from the Rust test (i.e. comparing against snapshot)...and then load that into a test in python (that checks the CFG still has edges, it won't without this fix). We don't have any code that does this yet AFAICS so this is a draft while that remains TODO.

fixes #1923

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.71%. Comparing base (2f588cb) to head (1d7b630).
Report is 184 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1924      +/-   ##
==========================================
- Coverage   83.78%   83.71%   -0.07%     
==========================================
  Files         196      196              
  Lines       37493    37538      +45     
  Branches    34306    34351      +45     
==========================================
+ Hits        31414    31426      +12     
- Misses       4296     4328      +32     
- Partials     1783     1784       +1     

Flag	Coverage Δ
python	92.34% <ø> (ø)
rust	82.91% <100.00%> (∅)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ 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.

[acl-cqc]

I think hugr-models arrival means we can drop this....