perf(simulator-specific): ⚡ Implement Rust-side percentage bonus calculations to replace GDScript logic

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

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

@@ -1385,6 +1385,22 @@ impl GdCity {
         self.inner.process_culture(&ty)
     }
 
+    /// Process culture accumulation with a percentage modifier added to the
+    /// raw culture gain. `total_pct` is the SUM of all percentage bonuses
+    /// (cult_pct + border_pct + difficulty_culture_mult - 1.0). Rust owns
+    /// both the raw gain and the percent-bonus application — no GDScript-side
+    /// post-call adjustment needed (warcouncil p1-39 Rail-1 port, 2026-04-27).
+    /// Returns true if border expansion is ready post-bonus.
+    #[func]
+    fn process_culture_with_modifier(&mut self, tile_yields_json: GString, total_pct: f64) -> bool {
+        let ty = Self::parse_tile_yields(&tile_yields_json.to_string());
+        let yields = self.inner.get_yields(&ty);
+        let raw_gain = yields.culture;
+        let scaled_gain = raw_gain * (1.0 + total_pct.max(-1.0));
+        self.inner.culture_stored += scaled_gain;
+        self.inner.can_expand()
+    }
+
     /// Add production production.
     #[func]
     fn add_production(&mut self, production: f64) {