crypto: Optimize line evaluation in BN254 pairings

[chfast]

Algebraic simplifications in the Jacobian add + line evaluation,
all recovering an intermediate reuse that was implicit in the original
reference but lost when the procedure was re-derived to fix the paper's
typos.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.97%. Comparing base (8a68f96) to head (f676b62).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1541   +/-   ##
=======================================
  Coverage   96.97%   96.97%           
=======================================
  Files         163      163           
  Lines       14455    14455           
  Branches     3385     3385           
=======================================
  Hits        14018    14018           
  Misses        307      307           
  Partials      130      130           

Flag	Coverage Δ
eest-develop	91.94% <100.00%> (ø)
eest-develop-gmp	26.53% <100.00%> (ø)
eest-legacy	17.55% <0.00%> (ø)
eest-libsecp256k1	28.17% <100.00%> (ø)
eest-stable	91.85% <100.00%> (ø)
evmone-unittests	92.64% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
core	96.02% <100.00%> (ø)
tooling	86.71% <ø> (ø)
tests	99.79% <ø> (ø)

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[rodiazet]

LGTM. Quite obvious simplification. Wondering why these papers don’t mention this. :)

[Copilot]

Pull request overview

This PR optimizes BN254 pairing line-evaluation computations by algebraically simplifying coefficient calculations and reusing intermediates in the Jacobian-add + line evaluation path, aligning with the referenced derivation notes.

Changes:

• Simplify t[0]/t[1] computations in lin_func_and_add() by reusing H and R (removing redundant multiplications).
• Factor common terms in lin_func() to reduce field multiplications.
• Minor comment correction/clarification in the Jacobian add derivation notes inline.

---

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

[chfast]

                                                   │ /proc/self/fd/11 │          /proc/self/fd/16          │
                                                   │      gas/s       │    gas/s     vs base               │
precompile<PrecompileId::ecpairing,_evmmax_cpp>-14        92.18M ± 1%   92.73M ± 0%  +0.59% (p=0.001 n=11)