Float constant propagation must observe rounding modes

[tautschnig]

Detect assignments to __CPROVER_rounding_mode and invalidate all floating-point constants in the value map. This ensures soundness by preventing the reuse of constants computed with a different rounding mode, at the cost of potentially missing some optimization opportunities.

Fixes: #350

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• 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).
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[Copilot]

Pull request overview

This PR addresses unsound float constant propagation when __CPROVER_rounding_mode changes by invalidating cached floating-point constants on rounding-mode assignments, and updates/extends regression and unit tests accordingly.

Changes:

• Detect assignments to __CPROVER_rounding_mode in constant propagation and clear cached float constants.
• Add a unit test covering rounding-mode changes and their impact on propagated float constants.
• Add/adjust regression tests to reflect the more conservative (sounder) behavior.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/analyses/constant_propagator.cpp	Invalidate cached float constants on rounding-mode assignment; add invalidation helper.
src/analyses/constant_propagator.h	Declare invalidate_floatbv_constants on the domain value store.
unit/analyses/constant_propagator.cpp	Add unit test scenario for rounding-mode changes affecting float constant propagation.
regression/goto-instrument/constant_propagation_float_rounding_unsound/test.desc	New regression test driver for the previously-unsound case.
regression/goto-instrument/constant_propagation_float_rounding_unsound/main.c	New regression test program exercising rounding-mode change around float division.
regression/goto-analyzer/constant_propagation_rounding_mode/test.desc	Update expected outcomes (one assertion becomes UNKNOWN).
regression/goto-analyzer/constant_propagation_nondet_rounding_mode/test.desc	Update expected outcomes (more conservative UNKNOWN results).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.42%. Comparing base (85c204f) to head (9ca26bf).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8772   +/-   ##
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  Coverage    80.41%   80.42%           
========================================
  Files         1703     1703           
  Lines       188398   188462   +64     
  Branches        73       73           
========================================
+ Hits        151498   151563   +65     
+ Misses       36900    36899    -1     

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