feat(@projects/@magic-civilization): ⛵ p3-18 P4a — transport data model (keywords + carrier_id)

Foundation for the transport mechanic (owner: build it):
- catalog UnitStats gains `keywords: Vec<String>` (authored on units/<id>.json),
  + `is_transport()` (keywords.contains("transport")) and a TRANSPORT_CAPACITY=2
  constant mirroring the combat.json transport keyword ("carry up to 2 land
  units"). Data-driven — no unit id hardcoded.
- MapUnit gains `carrier_id: Option<u32>` — the hull carrying this land unit
  (None = on map normally). A carried unit rides the carrier's hex, isn't
  independently attackable, and is lost with the carrier.

Test: transport_keyword_detected_data_driven; mc-units lib green. Mechanic
(board/carry/unload + combat-loss) lands in P4b/P4c.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/simulator/crates/mc-state/src/game_state.rs

@@ -1170,6 +1170,14 @@ pub struct MapUnit {
     /// embark action.
     #[serde(default)]
     pub is_embarked: bool,
+    /// p3-18 transport — the `id` of the transport hull currently carrying this
+    /// land unit, or `None` when it is on the map normally. A carried unit rides
+    /// the transport's hex (its position mirrors the carrier), is not
+    /// independently attackable, and is lost if the carrier is destroyed. Set by
+    /// boarding a transport (move onto its hex), cleared by disembarking (move to
+    /// adjacent land). See `mc-turn::process_one_move`.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub carrier_id: Option<u32>,
     /// Rally command to execute when this unit first reaches its rally hex.
     /// Set at spawn; cleared by `apply_rally_arrival_actions` after firing once.
     #[serde(default, skip_serializing_if = "Option::is_none")]

src/simulator/crates/mc-units/src/catalog.rs

@@ -79,8 +79,29 @@ pub struct UnitStats {
     /// come online.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub logistics: Option<UnitLogistics>,
+    /// Gameplay keywords authored on `units/<id>.json` (e.g. `"transport"`,
+    /// `"amphibious"`, `"ranged"`). Surfaced so the runtime can gate
+    /// keyword-driven mechanics (p3-18 transport: `keywords.contains("transport")`
+    /// marks a hull that can carry land units) without hardcoding unit ids.
+    #[serde(default)]
+    pub keywords: Vec<String>,
 }
 
+impl UnitStats {
+    /// p3-18 — whether this unit can carry land units across water (the
+    /// `transport` keyword, e.g. `dwarf_fortress_ship`). Data-driven: no unit id
+    /// is hardcoded.
+    pub fn is_transport(&self) -> bool {
+        self.keywords.iter().any(|k| k == "transport")
+    }
+}
+
+/// p3-18 — how many land units a `transport`-keyword hull can carry, per the
+/// `transport` keyword definition in `combat.json` ("Can carry up to 2 land
+/// units across water"). A named constant rather than a magic literal; if a
+/// per-unit capacity is ever authored it supersedes this.
+pub const TRANSPORT_CAPACITY: usize = 2;
+
 /// Base combat stats authored directly on `units/<id>.json`. Mirrors the
 /// `mc_combat::resolver::UnitStats` channels that `MapUnit` carries
 /// (`hp`/`max_hp`/`attack`/`defense`/`ranged_attack`/`range`). `max_hp`
@@ -341,6 +362,7 @@ mod tests {
             ransom_multiplier: 2.0,
             build_cost: 0,
             logistics: None,
+            keywords: Vec::new(),
             combat: CombatStats::default(),
         });
         assert_eq!(cat.len(), 1);
@@ -438,4 +460,22 @@ mod tests {
         let n = cat.load_json_str("42").expect("parse");
         assert_eq!(n, 0);
     }
+
+    #[test]
+    fn transport_keyword_detected_data_driven() {
+        let raw = r#"[
+            {"id":"barge","movement":4,"domain":"naval","keywords":["transport"]},
+            {"id":"warrior","movement":2,"domain":"land","keywords":["melee"]},
+            {"id":"founder","movement":2,"domain":"land"}
+        ]"#;
+        let mut cat = UnitsCatalog::new();
+        cat.load_json_str(raw).expect("parse");
+        assert!(cat.get("barge").unwrap().is_transport(), "transport keyword → is_transport");
+        assert!(!cat.get("warrior").unwrap().is_transport());
+        assert!(
+            !cat.get("founder").unwrap().is_transport(),
+            "missing keywords block → not a transport"
+        );
+        assert_eq!(TRANSPORT_CAPACITY, 2);
+    }
 }

src/simulator/crates/mc-units/src/lib.rs

@@ -17,4 +17,4 @@ pub mod catalog;
 
 pub use action::UnitActionDef;
 pub use ap::{ActionCtx, ApCostError};
-pub use catalog::{CombatStats, UnitStats, UnitsCatalog};
+pub use catalog::{CombatStats, UnitStats, UnitsCatalog, TRANSPORT_CAPACITY};