What are JSpecify nullability annotations in JUnit 6 and why do they matter?

// JUnit 6 API example with JSpecify annotations:
// (simplified from actual JUnit 6 source)

@NullMarked  // all unannotated types are non-null by default
public class Assertions {

    // @Nullable return type: assertNull can only be called with a
    // value the compiler considers potentially null
    public static void assertNull(@Nullable Object actual) { ... }

    // Non-null enforced: message MUST be non-null
    public static void assertEquals(
        Object expected,
        Object actual,
        String message  // @NonNull from @NullMarked
    ) { ... }
}

// Kotlin benefit: Kotlin compiler recognises JSpecify annotations
// Smart casts work after assertNotNull():
val result: String? = getValue()
assertNotNull(result)        // Kotlin sees NonNull contract
println(result.length)       // No !! needed -- compiler knows non-null

// JUnit 5 problem:
assertNotNull(result)        // JUnit 5 had no contract
println(result!!.length)     // !! still needed -- compiler didnt know