feat(hugr-llvm): Emit rounding operation for FloatOps::fround extension ops

[maximilianruesch]

Adds FloatOps::fround LLVM lowering support. This is to add support for rounding operations in Selene, connected to Quantinuum/guppylang#1561.

I am a bit unsure how the tests work, but simply adding another test case in line with the existing ones seems to work fine.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.83%. Comparing base (93aac28) to head (cfa51a8).

Files with missing lines	Patch %	Lines
hugr-llvm/src/extension/float.rs	80.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2949      +/-   ##
==========================================
- Coverage   83.84%   83.83%   -0.01%     
==========================================
  Files         267      267              
  Lines       52923    52933      +10     
  Branches    46854    46864      +10     
==========================================
+ Hits        44371    44379       +8     
  Misses       6283     6283              
- Partials     2269     2271       +2     

Flag	Coverage Δ
python	88.76% <ø> (ø)
rust	83.20% <80.00%> (-0.01%)	⬇️

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[acl-cqc]

Generally this looks fine, but I'm trying to understand the connection to Quantinuum/guppylang#1561. This PR implies that the current situation is that we can't compile rounding at all, even without an extra precision argument?

[acl-cqc]

The thing that worries me here is that fround appears to be unary, whereas all the others seem to be binary (taking two floats)....

[maximilianruesch]

This is not true, fneg is also unary and appears in the test cases.