crypto: Reuse V in the G2 point addition

[chfast]

Y3 re-expanded U1 * H_squared, the term already bound to V one line above
for X3. This is the canonical add-1998-cmo-2 formula, which defines V and
reuses it in Y3 = r·(V − X3) − S1·HHH — so it was never a different formula
variant, just one term written out longhand. V has been there since the
original commit.

#1542 ("Trivial reuse of computed values in pairing helpers") made this exact
substitution in the sibling lin_func_and_add but did not reach add(), which
is why the two disagreed. ecc::add reuses its v correctly too, so this was
the only one of the three copies still expanding the term.

Worth about 0.3% of the ECPAIRING instruction count — add() runs ~20 times per
pair through mul_by_X/g2_subgroup_check. Consistent with the +0.83% Mgas/s
#1542 measured for the same substitution in the hotter function, and confirmation
that the compiler does not CSE this away on its own.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Optimizes BN254 G2 point addition by reusing an existing intermediate value.

Changes:

• Reuses V when calculating Y3, avoiding a redundant field multiplication.
• Aligns add() with equivalent pairing and ECC formulas.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.71%. Comparing base (9888163) to head (d43c961).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1636   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
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  Files         171      171           
  Lines       15607    15607           
  Branches     3610     3610           
=======================================
  Hits        15251    15251           
  Misses        269      269           
  Partials       87       87           

Flag	Coverage Δ
eest-develop	88.62% <100.00%> (ø)
eest-develop-gmp	26.53% <100.00%> (ø)
eest-legacy	17.15% <0.00%> (ø)
eest-libsecp256k1	28.83% <100.00%> (ø)
eest-stable	88.62% <100.00%> (ø)
evmone-unittests	93.42% <100.00%> (ø)

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Components	Coverage Δ
core	96.10% <100.00%> (ø)
tooling	91.92% <ø> (ø)
tests	99.80% <ø> (ø)

Files with missing lines	Coverage Δ
lib/evmone_precompiles/pairing/bn254/utils.hpp	100.00% <100.00%> (ø)

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