Make stableNull public in the stdlib

docs/_docs/reference/experimental/explicit-nulls.md

@@ -457,6 +457,31 @@ See [more examples](https://github.com/scala/scala3/blob/main/tests/explicit-nul
 Currently, we are unable to track paths with a mutable variable prefix.
 For example, `x.a` if `x` is mutable.
 
+### Tracking Mutable Fields with `@stableNull`
+
+By default, mutable class fields (`var`) are not tracked by flow typing, because a concurrent
+assignment could set the field to `null` between the null check and its subsequent use.
+To opt into flow typing for a mutable field, annotate it with
+`scala.annotation.stableNull`.
+The field will then be tracked whenever it is accessed through a stable prefix.
+
+```scala
+import scala.annotation.stableNull
+
+class A:
+  @stableNull private var s: String | Null = null
+  def getS: String =
+    if s == null then s = ""
+    s // s: String
+```
+
+**Warning:** `@stableNull` can break null safety. The compiler assumes the field stays
+non-nullable after a null check, but nothing prevents another thread, a reentrant call, or
+unrelated code reachable from the same reference from reassigning it to `null` in between.
+We recommend only using it to manage a mutable nullable state *locally* within a class.
+A typical case is a field where `null` represents an uninitialized value that is assigned
+once on first access.
+
 ### Unsupported Idioms
 
 We don't support:

library/src/scala/annotation/stableNull.scala

@@ -7,4 +7,4 @@ package scala.annotation
  *  prefix is a stable path.
  *  See `tests/explicit-nulls/pos/force-track-var-fields.scala` for an example.
  */
-private[scala] final class stableNull extends StaticAnnotation
+final class stableNull extends StaticAnnotation