51x5 inverse (WIP)

[mitschabaude]

Summary

• Restores src/51x5/inverse.ts (Kaliski almost-inverse + Montgomery inverse) that was carved out of 51x5: single-field add/sub/reduce + tests + c51/c52 fix #20 to keep that PR focused on field ops.
• Marked WIP/untested in the source itself; needs tests before it's ready to land.

Stacked on feature/51x5-curve-ops because it depends on fieldWithMethods / FieldLayout / bigintToData from that branch. Retarget to main once #20 merges.

Test plan

• Property test makeOdd against a JS reference (u/2^k / s*2^k for k = ctz(u))
• Property test almostInverse — check the returned k and the invariant r * a ≡ ±2^k (mod p)
• End-to-end: inverse(scratch, r, a); multiply(r, r, a) === 2^K (Montgomery form of 1)
• Cross-check against FpBigint.inverse via createEquivalentWasm
• Edge cases: a = 1, a = p-1, a small (< 2^51)

🤖 Generated with Claude Code