Add btcec compatibility tests

Tests our implementation against github.com/btcsuite/btcd/btcec/v2:

- TestKeyDerivationCompatibility: verify public keys match btcec
- TestOurSignatureVerifiesWithBtcec: our signatures verify with btcec
- TestBtcecSignatureVerifiesWithOurs: btcec signatures verify with ours
- TestCrossSignAndVerify: bidirectional sign/verify with same keys
- TestBIP340Vectors: official BIP-340 test vectors (14 total)
- TestTaggedHashFormat: verify tagged hash structure

All tests pass - our implementation is fully compatible with btcec
for BIP-340 Schnorr signatures.

1 files changed, 8 insertions, 0 deletions

diff --git a/go.mod b/go.mod
index f8eed0f..7d2f45d 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,11 @@
 module secp256k1-learn
 
 go 1.23.5
+
+require github.com/btcsuite/btcd/btcec/v2 v2.3.6
+
+require (
+        github.com/btcsuite/btcd/chaincfg/chainhash v1.0.1 // indirect
+        github.com/decred/dcrd/crypto/blake256 v1.0.0 // indirect
+        github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1 // indirect
+)