Integrate standalone SPIN encoding into Silent OT

[ladnir]

Summary

• Integrate the standalone SPIN encoder through MultType::Spin, with internal parameter selection and generic XOR-based coefficient contexts.
• Reuse prepared codes for regular Silent OT; use banked heuristic refresh for stationary noise, preserving explicit seed and buffer preparation and deallocation through clear().
• Add the standard CMake fetch/install dependency path for SPIN 0.2. Enable SPIN by default with Silent OT or Silent VOLE; allow explicit opt-out.
• Pin the merged cryptoTools VAES support and optimize batched sender hashing. Keep benchmarks in the existing frontend.
• Register SPIN integration tests in the existing test collection and extend the existing CMake consumer check; no new SPIN test executables.
• Remove developer audit notes. The final diff contains no inherited DPF, RevCuckoo, or PPRF changes; those remain separate review work.

Validation

Full GitHub Actions CI passed on branch tip 10d1605263fa9d8f80738f4e9313133a77c050d2:
https://github.com/osu-crypto/libOTe/actions/runs/35872238773

• Ubuntu build, tests, and package-consumer checks
• macOS build and tests with SPIN disabled
• Windows build and tests
• Curve25519 sodium fallback

Focused Linux validation also passed frontend_libOTe -u Spin, regular/stationary hashed OT correctness, and the installed SPIN consumer check.