fix: randomize UUIDv7 rand_a on millisecond-resolution clocks

version7.go

@@ -91,8 +91,19 @@ func getV7Time() (milli, seq int64) {
 
 	nano := timeNow().UnixNano()
 	milli = nano / nanoPerMilli
-	// Sequence number is between 0 and 3906 (nanoPerMilli>>8)
-	seq = (nano - milli*nanoPerMilli) >> 8
+	if hasSubMilliClock {
+		// Sequence number is between 0 and 3906 (nanoPerMilli>>8)
+		seq = (nano - milli*nanoPerMilli) >> 8
+	} else {
+		// The platform's wall clock has only millisecond resolution (e.g. on
+		// wasm), so the sub-millisecond fraction above would always be zero.
+		// Fill the 12-bit rand_a field with random data instead, as permitted
+		// by RFC 9562. Ordering within a millisecond is still preserved by the
+		// lastV7time+1 step below.
+		var b [2]byte
+		randomBits(b[:])
+		seq = int64(b[0]&0x0f)<<8 | int64(b[1])
+	}
 	now := milli<<12 + seq
 	if now <= lastV7time {
 		now = lastV7time + 1

version7_other.go

@@ -0,0 +1,12 @@
+// Copyright 2026 Google Inc.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !js,!wasip1
+
+package uuid
+
+// hasSubMilliClock reports whether the platform's wall clock is guaranteed to
+// have sub-millisecond resolution. Non-wasm targets provide it, so v7 derives
+// the rand_a field from a real sub-millisecond timestamp fraction.
+const hasSubMilliClock = true

version7_wasm.go

@@ -0,0 +1,14 @@
+// Copyright 2026 Google Inc.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js wasip1
+
+package uuid
+
+// hasSubMilliClock reports whether the platform's wall clock is guaranteed to
+// have sub-millisecond resolution. On the wasm targets (js and wasip1) the
+// resolution is host-dependent and sub-millisecond precision is not
+// guaranteed, so v7 fills the rand_a field with random bits instead of a
+// timestamp fraction.
+const hasSubMilliClock = false