Optimize appendString: bulk array copy via String.getChars

Replace char-by-char appendUnsafeC loop with String.getChars bulk copy
directly into CharBuilder's backing array. This eliminates per-character
method call overhead and allows the JVM/native runtime to use optimized
memory copy intrinsics.

Upstream: He-Pin/sjsonnet@5e9686cd (appendString portion)

Author: He-Pin

sjsonnet/src/sjsonnet/BaseCharRenderer.scala

@@ -179,13 +179,14 @@ class BaseCharRenderer[T <: upickle.core.CharOps.Output](
     }
   }
 
+  /** Bulk-copy String chars into CharBuilder's backing array.
+    * Safe because ensureLength(len) guarantees capacity for currentLength + len.
+    */
   protected def appendString(s: String): Unit = {
     val len = s.length
-    var i = 0
     elemBuilder.ensureLength(len)
-    while (i < len) {
-      elemBuilder.appendUnsafeC(s.charAt(i))
-      i += 1
-    }
+    val pos = elemBuilder.getLength
+    s.getChars(0, len, elemBuilder.arr, pos)
+    elemBuilder.length = pos + len
   }
 }