perf: add toAffineX for x-only ECDH, benchmark ecdhXOnly path

- Add ECPoint.toAffineX that computes only x = X/Z² (saves 2M vs full
  toAffine which also computes Y/Z³)
- Use toAffineX in ecdhXOnly since only the x-coordinate is needed
- Add ecdhXOnly benchmark measuring the actual Nostr ECDH production
  path (Secp256k1Instance.pubKeyTweakMulCompact delegates to ecdhXOnly)
- Fix ktlint KDoc-inside-class-body violations

The old benchmark measured pubKeyTweakMul(02||x, key) which pays for:
array allocation, compressed key parsing (sqrt), full toAffine, and
re-serialization. ecdhXOnly avoids the array overhead and serialization.

https://claude.ai/code/session_01BhU63WUe9AhikZxRdw3Lpg

quartz/src/jvmTest/kotlin/com/vitorpamplona/quartz/utils/secp256k1/Secp256k1Benchmark.kt

@@ -267,11 +267,11 @@ class Secp256k1Benchmark {
                 },
             )
 
-        // --- pubKeyTweakMulCompact (the actual Secp256k1Instance pattern) ---
+        // --- pubKeyTweakMulCompact (old pattern via pubKeyTweakMul) ---
         val pub2xOnly = hexToBytes("c2f9d9948dc8c7c38321e4b85c8558872eafa0641cd269db76848a6073e69133")
         results +=
             bench(
-                name = "pubKeyTweakMulCompact",
+                name = "tweakMulCompact (old)",
                 warmup = 10,
                 iterations = 50,
                 nativeOp = { native.pubKeyTweakMul(h02 + pub2xOnly, privKey).copyOfRange(1, 33) },
@@ -284,6 +284,19 @@ class Secp256k1Benchmark {
                 },
             )
 
+        // --- ecdhXOnly (actual Nostr ECDH path) ---
+        results +=
+            bench(
+                name = "ecdhXOnly (Nostr)",
+                warmup = 10,
+                iterations = 50,
+                nativeOp = { native.pubKeyTweakMul(h02 + pub2xOnly, privKey).copyOfRange(1, 33) },
+                kotlinOp = {
+                    com.vitorpamplona.quartz.utils.secp256k1.Secp256k1
+                        .ecdhXOnly(pub2xOnly, privKey)
+                },
+            )
+
         // Print results
         println()
         println("=".repeat(90))