embed secp256k1: replace btcec with internal pure-go implementation

This removes all external dependencies by embedding the secp256k1-learn
implementation into internal/secp256k1.

Changes:
- Add internal/secp256k1 with field arithmetic, curve ops, keys, schnorr
- Update keys.go to use internal secp256k1 package
- Remove btcec/btcutil dependencies (go.mod is now clean)
- All tests pass

Tradeoffs:
- ~10x slower crypto ops vs btcec (acceptable for nostr use case)
- Not constant-time (documented limitation)
- Zero external dependencies

Refs: code.northwest.io/secp256k1-learn

1 files changed, 0 insertions, 8 deletions

diff --git a/go.mod b/go.mod
index e63be4c..5fd9f48 100644
--- a/go.mod
+++ b/go.mod
@@ -1,11 +1,3 @@
 module code.northwest.io/nostr
 
 go 1.25
-
-require github.com/btcsuite/btcd/btcec/v2 v2.3.4
-
-require (
-        github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0 // indirect
-        github.com/decred/dcrd/crypto/blake256 v1.1.0 // indirect
-        github.com/decred/dcrd/dcrec/secp256k1/v4 v4.4.0 // indirect
-)