crypto: Extract modexp_even()

[chfast]

No description provided.

[Copilot]

Pull request overview

This PR extracts the modular exponentiation logic for even moduli into a separate modexp_even() function, improving code organization and reusability. The extraction follows the same pattern as the existing modexp_odd() and modexp_pow2() helper functions.

Key changes:

• Extracted inline even-modulus logic from modexp_impl() into a new modexp_even() template function
• Simplified the modexp_impl() function by replacing ~20 lines of inline code with a single function call
• Added documentation referencing the Montgomery reduction algorithm paper

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[Copilot]

Consider adding an explanatory comment for the assertion, similar to the one in modexp_pow2 at line 77. This would clarify why k must be non-zero and improve code consistency.

Suggested change

	assert(k != 0);
	assert(k != 0); // k must be non-zero because the modulus has the form mod_odd * 2^k with k > 0; k == 0 is handled by the odd-modulus path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (6d91420) to head (e66a260).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1413      +/-   ##
==========================================
+ Coverage   78.93%   81.67%   +2.74%     
==========================================
  Files         151      152       +1     
  Lines       13543    13566      +23     
  Branches     3216     3217       +1     
==========================================
+ Hits        10690    11080     +390     
+ Misses        574      343     -231     
+ Partials     2279     2143     -136     

Flag	Coverage Δ
eest-develop	91.13% <100.00%> (?)
eest-develop-gmp	29.16% <100.00%> (?)
eest-legacy	15.35% <0.00%> (-0.01%)	⬇️
eest-legacy-silkpre	24.50% <85.71%> (+0.01%)	⬆️
evmone-unittests	76.21% <85.71%> (+<0.01%)	⬆️

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

Components	Coverage Δ
core	94.42% <100.00%> (+6.89%)	⬆️
tooling	83.63% <ø> (+3.67%)	⬆️
tests	73.52% <ø> (ø)

Files with missing lines	Coverage Δ
lib/evmone_precompiles/modexp.cpp	100.00% <100.00%> (+9.21%)	⬆️

... and 21 files with indirect coverage changes

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