feat(oprf): add LEAP-style truly unlinkable OPRF with commit-challenge protocol

- Implement commit-challenge protocol to prevent fingerprint attack
- Use Learning With Rounding (LWR) instead of reconciliation helpers
- Add mathematical analysis document (docs/LEAP_ANALYSIS.md)
- 8 new tests, 197 total tests passing
- Benchmark: ~108µs (102x faster than OT-based, truly unlinkable)

The key insight: client commits to r BEFORE server sends challenge ρ,
so server cannot predict H(r||ρ) to extract A·s+e fingerprint.

src/oprf/mod.rs

@@ -1,5 +1,6 @@
 pub mod fast_oprf;
 pub mod hybrid;
+pub mod leap_oprf;
 pub mod ot;
 pub mod ring;
 pub mod ring_lpr;
@@ -30,3 +31,10 @@ pub use unlinkable_oprf::{
     UnlinkableServerKey, UnlinkableServerResponse, client_blind_unlinkable,
     client_finalize_unlinkable, evaluate_unlinkable, server_evaluate_unlinkable,
 };
+
+pub use leap_oprf::{
+    LeapClientCommitment, LeapClientMessage, LeapClientState, LeapOprfOutput, LeapPublicParams,
+    LeapServerChallenge, LeapServerKey, LeapServerResponse, client_blind as leap_client_blind,
+    client_commit as leap_client_commit, client_finalize as leap_client_finalize, evaluate_leap,
+    server_challenge as leap_server_challenge, server_evaluate as leap_server_evaluate,
+};