feat(@projects/@magic-civilization): ⛵ p3-18 P4c — carried units lost when their transport is destroyed

After the PvP combat phase, prune_orphaned_cargo drops any unit whose carrier_id
references a hull no longer in its player's roster — the transport sank, its
cargo goes down with it. Order-preserving removal that keeps the index-parallel
unit_upkeep aligned; idempotent and a no-op for carrier-free rosters (so existing
combat is untouched — pvp tests still green).

Test: destroyed_transport_loses_cargo (orphaned cargo pruned, uncarried unit
survives, unit_upkeep stays aligned).

Known follow-up: carried units ride the hull's hex (stacked); fully shielding
them from being individually targeted needs combat target-selection changes that
touch 1UPT assumptions — tracked, not in this commit.

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

src/simulator/crates/mc-turn/src/processor.rs

@@ -3601,6 +3601,10 @@ impl TurnProcessor {
 
         // p2-55: drain capture/ransom/destroyed events into the TurnResult.
         state.pending_capture_events.drain_into(result);
+
+        // p3-18 transport — units whose carrier hull was destroyed this phase go
+        // down with it (lost at sea).
+        prune_orphaned_cargo(state);
     }
 
     // ── Phase 5c: City siege ──────────────────────────────────────────────
@@ -4706,6 +4710,30 @@ fn embark_level_for(player: &crate::game_state::PlayerState) -> mc_core::EmbarkL
     player.embark_level
 }
 
+/// p3-18 transport — drop any unit whose `carrier_id` references a hull that no
+/// longer exists in its player's roster (the transport was destroyed). Carried
+/// units go down with the ship. Order-preserving (removes descending indices)
+/// and keeps the index-parallel `unit_upkeep` aligned. Idempotent — safe to call
+/// after any combat phase.
+fn prune_orphaned_cargo(state: &mut crate::game_state::GameState) {
+    for player in state.players.iter_mut() {
+        let live: std::collections::HashSet<u32> = player.units.iter().map(|u| u.id).collect();
+        let drop: Vec<usize> = player
+            .units
+            .iter()
+            .enumerate()
+            .filter(|(_, u)| matches!(u.carrier_id, Some(cid) if !live.contains(&cid)))
+            .map(|(i, _)| i)
+            .collect();
+        for &i in drop.iter().rev() {
+            player.units.remove(i);
+            if i < player.unit_upkeep.len() {
+                player.unit_upkeep.remove(i);
+            }
+        }
+    }
+}
+
 fn process_one_move(state: &mut GameState, req: &crate::game_state::MoveRequest) -> MoveOutcome {
     use mc_pathfinding::{find_path, UnitDomain};
 
@@ -5179,6 +5207,29 @@ mod move_request_tests {
         assert_eq!(state.players[0].units[0].carrier_id, None, "warrior stayed off the full hull");
     }
 
+    #[test]
+    fn destroyed_transport_loses_cargo() {
+        // carrier id 2 is absent from the roster (destroyed); its cargo (id 1)
+        // must be pruned, while an uncarried unit (id 3) survives.
+        let mut state = transport_state();
+        state.players.push(PlayerState {
+            player_index: 0,
+            units: vec![
+                MapUnit {
+                    id: 1, col: 2, row: 0, unit_id: "warrior".into(),
+                    carrier_id: Some(2), ..MapUnit::default()
+                },
+                MapUnit { id: 3, col: 0, row: 0, unit_id: "warrior".into(), ..MapUnit::default() },
+            ],
+            unit_upkeep: vec![1, 1],
+            ..PlayerState::default()
+        });
+        prune_orphaned_cargo(&mut state);
+        let ids: Vec<u32> = state.players[0].units.iter().map(|u| u.id).collect();
+        assert_eq!(ids, vec![3], "orphaned cargo lost with the sunk hull; uncarried unit survives");
+        assert_eq!(state.players[0].unit_upkeep.len(), 1, "unit_upkeep stays index-aligned");
+    }
+
     #[test]
     fn zero_budget_rejects() {
         let mut state = build_state_with_unit(7, (0, 0), 0, |_, _| "plains");