feat(@projects/@magic-civilization): 🗺️ p3-29 (2) — surface CityBordersExpanded as a turn event

Second p3-29 step-1 event: the Rust turn emits border expansion (process_culture now takes
&mut events, collects claimed tiles + flushes TurnEvent::CityBordersExpanded with clan/city/hex)
— single-source replacement for the GDScript turn's inline `city_border_expanded` signal.
Surfaced through all four TurnEvent consumers + tested (culture_expansion_claims_frontier_tiles
now asserts the event). Events-only → no state change → golden/combat unaffected. 2/3 step-1
events done (CityGrew, CityBordersExpanded); FloraSuccession next.

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

src/simulator/api-gdext/src/replay.rs

@@ -210,6 +210,14 @@ fn event_to_dict(evt: &TurnEvent) -> Dictionary {
             d.set("col", hex.q as i64);
             d.set("row", hex.r as i64);
         }
+        TurnEvent::CityBordersExpanded { turn, clan, city, hex } => {
+            d.set("kind", GString::from("CityBordersExpanded"));
+            d.set("turn", *turn as i64);
+            d.set("clan", clan.0 as i64);
+            d.set("city", GString::from(city.0.as_str()));
+            d.set("col", hex.q as i64);
+            d.set("row", hex.r as i64);
+        }
         TurnEvent::GameOver { turn, winner, reason_kind, condition, resigned_clan } => {
             d.set("kind", GString::from("GameOver"));
             d.set("turn", *turn as i64);

src/simulator/crates/mc-player-api/src/dispatch.rs

@@ -979,9 +979,10 @@ fn translate_processor_events(events: &[mc_replay::TurnEvent]) -> Vec<Event> {
             | mc_replay::TurnEvent::EnvelopeIntercepted { .. }
             | mc_replay::TurnEvent::LinkSevered { .. }
             | mc_replay::TurnEvent::LinkRestored { .. }
-            // p3-29: surfaced for replay + the live UI (city growth), not the
+            // p3-29: surfaced for replay + the live UI (city growth / border
-            // wire protocol — drop here to keep the match exhaustive.
+            // expansion), not the wire protocol — drop here to keep it exhaustive.
-            | mc_replay::TurnEvent::CityGrew { .. } => {}
+            | mc_replay::TurnEvent::CityGrew { .. }
+            | mc_replay::TurnEvent::CityBordersExpanded { .. } => {}
         }
     }
     out

src/simulator/crates/mc-replay/src/event.rs

@@ -121,6 +121,18 @@ pub enum TurnEvent {
         /// Hex the city sits on, for the viewer to centre on.
         hex: TileCoord,
     },
+    /// p3-29: a city expanded its borders, claiming a new tile (single-source
+    /// replacement for the GDScript turn's inline `city_border_expanded` signal).
+    CityBordersExpanded {
+        /// Turn the event fired on.
+        turn: u32,
+        /// Owning clan.
+        clan: ClanId,
+        /// Synthesised city name (`city_<pi>_<idx>`).
+        city: CityName,
+        /// The newly-claimed tile.
+        hex: TileCoord,
+    },
     /// A wonder finished construction.
     WonderBuilt {
         /// Turn the event fired on.
@@ -514,6 +526,7 @@ impl TurnEvent {
             | Self::UnitCreated { turn, .. }
             | Self::CityBuildingCompleted { turn, .. }
             | Self::CityGrew { turn, .. }
+            | Self::CityBordersExpanded { turn, .. }
             | Self::WonderBuilt { turn, .. }
             | Self::WarDeclared { turn, .. }
             | Self::PeaceSigned { turn, .. }

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

@@ -434,7 +434,7 @@ impl TurnProcessor {
             self.process_city_production(state, pi, &mut result.events_emitted);
             self.try_found_city(state, pi, &mut result.events_emitted);
             self.try_spawn_unit(state, pi, &mut result);
-            self.process_culture(state, pi);
+            self.process_culture(state, pi, &mut result.events_emitted);
             self.process_culture_research(state, pi);
             self.process_science(state, pi, &mut result.events_emitted);
         }
@@ -1129,7 +1129,12 @@ impl TurnProcessor {
         }
     }
 
-    fn process_culture(&self, state: &mut GameState, pi: usize) {
+    fn process_culture(
+        &self,
+        state: &mut GameState,
+        pi: usize,
+        events: &mut Vec<mc_replay::TurnEvent>,
+    ) {
         // Grid dims read before the &mut player borrow (needed for in-bounds
         // candidate filtering during border expansion).
         let (gw, gh) = state
@@ -1137,6 +1142,10 @@ impl TurnProcessor {
             .as_ref()
             .map(|g| (g.width, g.height))
             .unwrap_or((0, 0));
+        let turn = state.turn;
+        // p3-29: tiles claimed this turn (city_idx, tile), flushed as
+        // `TurnEvent::CityBordersExpanded` so the live UI + replay see expansion.
+        let mut expanded: Vec<(usize, (i32, i32))> = Vec::new();
         let player = &mut state.players[pi];
         let culture_axis = *player.strategic_axes.get("culture").unwrap_or(&2);
         let per_city_yield = culture_axis as f64 * Self::BENCH_CULTURE_PER_AXIS_POINT;
@@ -1197,11 +1206,22 @@ impl TurnProcessor {
                     ot.push(pick);
                     claimed.insert(pick);
                     player.culture_pool.consume_expansion(ci);
+                    expanded.push((ci, pick));
                 }
             }
         }
 
         player.culture_total = player.culture_pool.culture_total;
+
+        // p3-29: flush border-expansion events (player no longer borrowed below).
+        for (ci, tile) in expanded {
+            events.push(mc_replay::TurnEvent::CityBordersExpanded {
+                turn,
+                clan: mc_replay::ClanId(pi as u32),
+                city: mc_replay::CityName(format!("city_{}_{}", pi, ci)),
+                hex: mc_replay::TileCoord::new(tile.0, tile.1),
+            });
+        }
     }
 
     // ── Phase 1b': Culture-research accumulation ────────────────────────
@@ -2677,7 +2697,7 @@ impl TurnProcessor {
             self.process_city_production(state, pi, &mut result.events_emitted);
             self.try_found_city(state, pi, &mut result.events_emitted);
             self.try_spawn_unit(state, pi, &mut result);
-            self.process_culture(state, pi);
+            self.process_culture(state, pi, &mut result.events_emitted);
             self.process_culture_research(state, pi);
             self.process_science(state, pi, &mut result.events_emitted);
         }
@@ -5837,10 +5857,19 @@ mod tests {
         state.players.push(p);
 
         let processor = TurnProcessor::new(1);
+        let mut events: Vec<mc_replay::TurnEvent> = Vec::new();
         for _ in 0..80 {
-            processor.process_culture(&mut state, 0);
+            processor.process_culture(&mut state, 0, &mut events);
         }
 
+        // p3-29: each border claim surfaces a CityBordersExpanded event.
+        assert!(
+            events
+                .iter()
+                .any(|e| matches!(e, mc_replay::TurnEvent::CityBordersExpanded { .. })),
+            "CityBordersExpanded must fire when borders expand; got {events:?}"
+        );
+
         let owned = &state.players[0].cities[0].owned_tiles;
         assert!(
             owned.contains(&(5, 5)),

src/simulator/crates/mc-turn/tests/event_collector_wiring.rs

@@ -207,6 +207,7 @@ fn ten_turn_run_emits_each_wired_variant() {
         .map(|e| match e {
             TurnEvent::CityBuildingCompleted { .. } => "CityBuildingCompleted",
             TurnEvent::CityGrew { .. } => "CityGrew",
+            TurnEvent::CityBordersExpanded { .. } => "CityBordersExpanded",
             TurnEvent::CityFounded { .. } => "CityFounded",
             TurnEvent::WonderBuilt { .. } => "WonderBuilt",
             TurnEvent::CityCaptured { .. } => "CityCaptured",
@@ -324,6 +325,7 @@ fn events_emitted_appears_on_turn_result() {
         .map(|e| match e {
             TurnEvent::CityBuildingCompleted { .. } => "CityBuildingCompleted",
             TurnEvent::CityGrew { .. } => "CityGrew",
+            TurnEvent::CityBordersExpanded { .. } => "CityBordersExpanded",
             TurnEvent::CityFounded { .. } => "CityFounded",
             TurnEvent::WonderBuilt { .. } => "WonderBuilt",
             TurnEvent::CityCaptured { .. } => "CityCaptured",