feat(protocol): Batch Validity (#187)

### Description

Part of a port migrating the batch types from `kona-derive` to
`op-alloy`. See
[`kona#695`](anton-rs/kona#695).

This PR introduces the `BatchValidity` type.

crates/protocol/src/batch/mod.rs

@@ -2,3 +2,6 @@
 
 mod r#type;
 pub use r#type::*;
+
+mod validity;
+pub use validity::BatchValidity;

crates/protocol/src/batch/validity.rs

@@ -0,0 +1,53 @@
+//! Contains the [BatchValidity] and its encodings.
+
+/// Batch Validity
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BatchValidity {
+    /// The batch is invalid now and in the future, unless we reorg, so it can be discarded.
+    Drop,
+    /// The batch is valid and should be processed
+    Accept,
+    /// We are lacking L1 information until we can proceed batch filtering
+    Undecided,
+    /// The batch may be valid, but cannot be processed yet and should be checked again later
+    Future,
+    /// Introduced in Holocene, a special variant of the `Drop` variant that signals not to flush
+    /// the active batch and channel, in the case of processing an old batch
+    Past,
+}
+
+impl BatchValidity {
+    /// Returns whether the batch is accepted.
+    pub const fn is_accept(&self) -> bool {
+        matches!(self, Self::Accept)
+    }
+
+    /// Returns whether the batch is dropped.
+    pub const fn is_drop(&self) -> bool {
+        matches!(self, Self::Drop)
+    }
+
+    /// Returns whether the batch is outdated.
+    pub const fn is_outdated(&self) -> bool {
+        matches!(self, Self::Past)
+    }
+
+    /// Returns whether the batch is future.
+    pub const fn is_future(&self) -> bool {
+        matches!(self, Self::Future)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_batch_validity() {
+        assert!(BatchValidity::Accept.is_accept());
+        assert!(BatchValidity::Drop.is_drop());
+        assert!(BatchValidity::Past.is_outdated());
+        assert!(BatchValidity::Future.is_future());
+    }
+}

crates/protocol/src/lib.rs

@@ -10,7 +10,7 @@
 extern crate alloc;
 
 mod batch;
-pub use batch::{BatchType, SINGLE_BATCH_TYPE, SPAN_BATCH_TYPE};
+pub use batch::{BatchType, BatchValidity, SINGLE_BATCH_TYPE, SPAN_BATCH_TYPE};
 
 mod block;
 pub use block::{BlockInfo, FromBlockError, L2BlockInfo};