feat: migrate secp256k1 from LongArray to Fe4/Wide8 structs

Full migration of the secp256k1 library from LongArray(4)/LongArray(8)
to Fe4/Wide8 struct types with @JvmField named Long fields. This
eliminates all array bounds checks from the hot path.

Files migrated (13 source + 7 test + 2 benchmark):
  - U256.kt, FieldP.kt, ScalarN.kt, Glv.kt, ECPoint.kt
  - FieldMulPlatform.kt (expect + 3 actuals), FieldMulFused.kt
  - PointTypes.kt (MutablePoint, AffinePoint, PointScratch)
  - KeyCodec.kt, Secp256k1.kt
  - All test files and benchmarks

Bytecode impact:
  Before: 464 laload/lastore (bounds-checked) in core arithmetic
  After:  0 laload/lastore, all getfield/putfield (no checks)

The public API (Secp256k1 object) is unchanged - it still accepts
and returns ByteArray. Fe4 conversion happens at the API boundary
via U256.fromBytes()/U256.toBytes().

All secp256k1 unit tests pass on JVM.

https://claude.ai/code/session_01Sxi6Gpxbstuj3Y8TBY7XrU

quartz/src/androidMain/kotlin/com/vitorpamplona/quartz/utils/secp256k1/FieldMulPlatform.android.kt

@@ -43,18 +43,18 @@ package com.vitorpamplona.quartz.utils.secp256k1
  * - The fused function stays within ART's inlining budget
  */
 internal actual fun fieldMulReduce(
-    out: LongArray,
-    a: LongArray,
-    b: LongArray,
-    w: LongArray,
+    out: Fe4,
+    a: Fe4,
+    b: Fe4,
+    w: Wide8,
 ) {
     fieldMulReduceWith(out, a, b, w) { x, y -> unsignedMultiplyHighFallback(x, y) }
 }
 
 internal actual fun fieldSqrReduce(
-    out: LongArray,
-    a: LongArray,
-    w: LongArray,
+    out: Fe4,
+    a: Fe4,
+    w: Wide8,
 ) {
     fieldSqrReduceWith(out, a, w) { x, y -> unsignedMultiplyHighFallback(x, y) }
 }