FpVar::conditional_enforce_equal accepts unequal constants #213

Summary of Bug

FpVar::conditional_enforce_equal returns success for every constant/constant pair
without checking either value or the guard. Unequal constants with a dynamically
allocated true guard therefore add no equality obligation, and the false implication
can be satisfied and proved.

Expected behavior is !condition || left == right. Equal constants and a false guard
must remain valid, while unequal constants under a true guard must be rejected.

Open PR #196 changes the Boolean
implementation only. This report concerns the distinct FpVar constant/constant branch
in src/fields/fp/mod.rs.

Version

• ark-r1cs-std commit
  45e4e2697626a9f9481fe57e70ed29c687194102
• Reproduced on Ubuntu with Rust 1.85.1 and default ark-r1cs-std features
  (std).

Steps to Reproduce

1. Check out the isolated
   reproduction commit.

2. Run:

   cd reproductions/field-conditional-equality
   cargo +1.85.1 test --locked --test field_conditional_equality \
     unequal_constants_must_force_public_condition_false -- --exact --nocapture

3. The test verifies a proof for unequal constants under public condition=true.
   Disabled, equal, mixed-variable, explicit-implication, and matrix-shape controls behave
   as expected.

CI: run 29434225152,
job 87416653404.