chore: bootstrap nana-story M0

crates/nana-engine/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "nana-engine"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+
+[dependencies]
+nana-domain.workspace = true
+thiserror.workspace = true
+
+[lints]
+workspace = true

crates/nana-engine/src/lib.rs

@@ -0,0 +1,48 @@
+use nana_domain::{RuntimeState, StateDelta};
+use thiserror::Error;
+
+#[derive(Debug, Error, PartialEq, Eq)]
+pub enum ReduceError {
+    #[error("state delta is not implemented yet: {0} operation(s)")]
+    NotImplemented(usize),
+}
+
+/// M0 contract seam. M1 replaces this placeholder with the only authoritative
+/// state mutation path in the application.
+pub fn apply_delta(state: &RuntimeState, delta: &StateDelta) -> Result<RuntimeState, ReduceError> {
+    if delta.ops.is_empty() {
+        Ok(state.clone())
+    } else {
+        Err(ReduceError::NotImplemented(delta.ops.len()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::BTreeMap;
+
+    use nana_domain::{RuntimeState, StateDelta};
+
+    use super::apply_delta;
+
+    #[test]
+    fn empty_delta_is_identity() {
+        let state = RuntimeState {
+            story_id: "story_demo".to_owned(),
+            current_node: "node_001".to_owned(),
+            current_branch: "branch_main".to_owned(),
+            world_flags: BTreeMap::new(),
+            relationships: BTreeMap::new(),
+            relationship_states: Vec::new(),
+            promises: Vec::new(),
+            knowledge: Vec::new(),
+            items: Vec::new(),
+            clocks: Vec::new(),
+            checks: Vec::new(),
+        };
+        assert_eq!(
+            apply_delta(&state, &StateDelta { ops: Vec::new() }),
+            Ok(state)
+        );
+    }
+}