Rewrite byte_extract from multi-dimensional array

[tautschnig]

As a follow-up to 78839a9: add support for rewriting multiple-of-element size access to arrays when working with multi-dimensional arrays.

Fixes: #8796

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.48%. Comparing base (cd64a96) to head (d5b671e).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8705   +/-   ##
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  Coverage    80.47%   80.48%           
========================================
  Files         1704     1704           
  Lines       188762   188774   +12     
  Branches        73       73           
========================================
+ Hits        151908   151936   +28     
+ Misses       36854    36838   -16     

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[tautschnig]

Now includes a minified version of the sample provided in #8796 to confirm the desired effect of the simplification improvement.

[rod-chapman]

Can we push this to the top of the queue to be reviewed please? This is blocking progress on mldsa-native.

[rod-chapman]

Regression testing with mlkem-native and mldsa-native (all parameter sets) is good at the HEAD of this branch now.

I have also tested the specific test case added for Issue #8796 with good results.

Please rebase and merge.

[Copilot]

Pull request overview

This PR improves handling of byte-level extraction and array rewrites for multi-dimensional arrays, and fixes SMT array-theory behavior for let-bound arrays by ensuring fresh let symbols remain connected to their bound arrays.

Changes:

• Enhance get_subexpression_at_offset to better decompose scaled offsets for multi-dimensional array layouts.
• In the bit-blaster, record array equalities for let bindings over unbounded arrays so element constraints propagate through the fresh symbol.
• Add regression tests for SMT2 let over arrays and a DFCC performance/regression scenario involving multi-dimensional arrays.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/util/pointer_offset_size.cpp	Adds a new decomposition path for scaled offsets into multi-dimensional arrays; factors out elem_size_bytes.
src/solvers/flattening/boolbv_let.cpp	Adds element-wise connection between fresh let symbols and bound unbounded arrays via recorded array equality.
regression/smt2_solver/let-array/let-array.smt2	New SMT2 regression demonstrating required propagation through let-bound arrays.
regression/smt2_solver/let-array/let-array.desc	Expected output for the new SMT2 regression.
regression/contracts-dfcc/multi_dimensional_array_performance/test.desc	New DFCC regression entry for the multi-dimensional array scenario.
regression/contracts-dfcc/multi_dimensional_array_performance/program.desc	Checks produced program output matches expected byte_extract pattern.
regression/contracts-dfcc/multi_dimensional_array_performance/main.c	New C harness intended to exercise the multi-dimensional array rewrite path under DFCC.

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