Migrate ChildrenTag.Root to be a weird value

[JakeWharton]

Like 100,000. This will prevent its accidental use with regular widgets whose children are likely to be numbered with the current value of 1.

Unfortunately we need to do a deprecation cycle here, as guest code will be expecting the value to be 1.

redwood/redwood-protocol/src/commonTest/kotlin/app/cash/redwood/protocol/ProtocolTest.kt

Lines 36 to 41 in e3e6281

	@Test fun constants() {
	// This is otherwise a change-detector test, but since these values are included in
	// the serialized form they must never change.
	assertThat(Id.Root.value).isEqualTo(0)
	assertThat(ChildrenTag.Root.value).isEqualTo(1)
	}