All-zero projective values compare equal to finite points #212

Summary of Bug

The short-Weierstrass ProjectiveVar accepts (x, y, z) = (0, 0, 0).
value() decodes any z = 0 value as the group identity, but gadget equality only
checks homogeneous cross-products. The all-zero value therefore compares equal to every
finite projective point, including the generator.

Expected behavior is equality of the decoded native group values. The canonical gadget
identity (0, 1, 0) and native projective equality both distinguish the identity from
a finite point.

Open PR #211 changes the fallback
subgroup check in the same file; it does not validate the all-zero representation or
change projective equality.

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/curve-relation-binding
   cargo +1.85.1 test --locked --test projective_relation_binding -- --nocapture

3. The test shows that public coordinates [0, 0, 0] decode as identity while
   satisfying equality with the finite generator. A Groth16 proof verifies; canonical
   identity, native equality, corrected-rule, and valid-point controls behave as expected.

CI: run 29424343134,
job 87382727104.