Fix uniqueThenTime to use most significant bits for time ordering

The previous implementation used the lower 44 bits of timeCounterBits,
which only contained the lower 24 bits of the millisecond timestamp.
This caused time wrapping every ~4.66 hours, breaking lexicographical
sorting.

Now uses the most significant 44 bits (timeCounter >> 20) to preserve
long-term time ordering as intended.

Co-authored-by: simbo1905 <322608+simbo1905@users.noreply.github.com>

Author: Copilot

json-java21/src/main/java/jdk/sandbox/demo/UUIDGenerator.java

@@ -66,12 +66,13 @@ public static UUID timeThenRandom() {
     /// └──────────────────────────────────────────────────────────────────────────────┘
     public static UUID uniqueThenTime(long uniqueMsb) {
         final int timeBits = 44;
-        final long timeMask = (1L << timeBits) - 1;
         final int randomBits = 20;
         final int randomMask = (1 << randomBits) - 1;
         long timeCounter = timeCounterBits();
         long msb = uniqueMsb;
-        long lsb = ((timeCounter & timeMask) << randomBits) | (getRandom().nextInt() & randomMask);
+        // Take the most significant 44 bits of timeCounter to preserve time ordering
+        long timeComponent = timeCounter >> (64 - timeBits); // timeBits is 44
+        long lsb = (timeComponent << randomBits) | (getRandom().nextInt() & randomMask);
         return new UUID(msb, lsb);
     }