Fix ROUND_TO_AWAY rounding decision in float_bvt

[tautschnig]

float_bvt::fraction_rounding_decision used
or_exprt(rounding_bit, sticky_bit)
for ROUND_TO_AWAY, which incorrectly rounds away from zero whenever any extra bits are non-zero. The correct formula is just rounding_bit, matching float_utilst.

ROUND_TO_AWAY means 'round to nearest, ties away from zero':

• rounding_bit=0: closer to truncated value, don't increment
• rounding_bit=1, sticky=0: exact tie, round away (increment)
• rounding_bit=1, sticky=1: closer to incremented value, increment

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[Copilot]

Pull request overview

Fixes the ROUND_TO_AWAY (round-to-nearest, ties away from zero) rounding increment decision in the float_bvt encoding to match IEEE-754 semantics and float_utilst, and adds a regression test for both SAT (--floatbv) and SMT2/Z3 paths.

Changes:

• Correct float_bvt::fraction_rounding_decision for ROUND_TO_AWAY to use only rounding_bit (not rounding_bit || sticky_bit).
• Add a new regression test Float-round-to-away covering tie and non-tie cases.
• Add an SMT2/Z3 regression descriptor to exercise the SMT path.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/solvers/floatbv/float_bv.cpp	Fixes the rounding increment condition for ROUND_TO_AWAY in the floatbv encoding.
regression/cbmc/Float-round-to-away/main.c	Adds a targeted regression program for ROUND_TO_AWAY tie/non-tie behavior.
regression/cbmc/Float-round-to-away/test.desc	Runs the regression with --floatbv (SAT/bit-vector path).
regression/cbmc/Float-round-to-away/test_smt.desc	Runs the regression with --smt2 --z3 (SMT path).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.63%. Comparing base (4ed6b6d) to head (136b9eb).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8983      +/-   ##
===========================================
+ Coverage    80.59%   80.63%   +0.04%     
===========================================
  Files         1713     1713              
  Lines       189403   189403              
  Branches        73       73              
===========================================
+ Hits        152647   152732      +85     
+ Misses       36756    36671      -85     

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