perf(mod-builder): store vars during preflight to avoid recomputation in trace fill

[jonathanpwang]

Previously, run_field_expression was called twice - once during preflight and again during trace filling - causing expensive vars computation to be duplicated.

This change:

• Extends FieldExpressionCoreRecordMut to store computed vars
• Updates preflight to save vars in the record after computation
• Updates trace filler to read precomputed vars from the record
• Adds generate_subrow_with_precomputed_vars to FieldExpr
• Extracts derive_flags_from_opcode helper for reuse

The vars (intermediate field expression results) are now computed once during preflight and reused during trace generation, eliminating redundant BigUint arithmetic operations.

[Copilot]

Pull request overview

This PR implements a performance optimization for the mod-builder circuit by storing intermediate field expression variables (vars) during preflight execution and reusing them during trace generation, eliminating redundant BigUint arithmetic operations.

Changes:

• Extends FieldExpressionCoreRecordMut to include var_limbs field for storing precomputed intermediate variables
• Updates preflight execution to compute and store vars in the record
• Modifies trace filler to read precomputed vars instead of recomputing them
• Adds generate_subrow_with_precomputed_vars method to FieldExpr that accepts precomputed vars
• Extracts derive_flags_from_opcode helper function for code reuse

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
crates/circuits/mod-builder/src/core_chip.rs	Core changes implementing vars storage in preflight and retrieval during trace filling; adds derive_flags_from_opcode helper
crates/circuits/mod-builder/src/builder.rs	Adds generate_subrow_with_precomputed_vars method that skips var computation and uses precomputed values
crates/circuits/mod-builder/src/tests.rs	Updates test infrastructure with increased buffer sizes, num_variables parameter, and switches to BabyBearPoseidon2Engine

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The change from BabyBearBlake3Engine to BabyBearPoseidon2Engine appears to be unrelated to the performance optimization described in the PR. Consider submitting this as a separate change or documenting it in the PR description if it's intentional.