perf: switch SW_EC to projective coordinates - guest-libs

[Tuanlinh12312]

Resolves INT-6136.

Update k256, p256, and pairing guest libraries to use projective coordinates:

• Add z coordinate to all point generators (z=1)
• Normalize before extracting affine coordinates (x_be_bytes, y_be_bytes, AffineCoordinates, ToEncodedPoint, to_affine)
• Use cross-multiplication for projective ConstantTimeEq
• Select all 3 coordinates in ConditionallySelectable
• Remove DefaultIsZeroes (identity is (0,1,0), not all zeros)
• Switch G2 pairing modules from impl_sw_affine! to impl_sw_proj!
• Add .normalize() in tests before comparing to known affine values