feat(@mc-ai): ✨ add trailing ai detection logic

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

src/simulator/crates/mc-ai/src/tactical/movement.rs

@@ -123,7 +123,7 @@ pub(crate) fn decide_movement(
                         decide_military_action(
                             unit, me, &enemy_units, &enemy_city_positions,
                             own_mil_count, enemy_mil_count, weights, rng,
-                            state.difficulty_threshold_mult,
+                            state.difficulty_threshold_mult, is_trailing,
                         )
                     } else {
                         None // deployed but no target: hold
@@ -142,6 +142,7 @@ pub(crate) fn decide_movement(
                         weights,
                         rng,
                         state.difficulty_threshold_mult,
+                        is_trailing,
                     );
                     if primary.is_some() {
                         primary
@@ -385,6 +386,35 @@ fn count_military(units: &[TacticalUnit]) -> usize {
     units.iter().filter(|u| is_military(u)).count()
 }
 
+/// Whether `me` is the trailing AI on this turn.
+///
+/// `true` iff (a) at least one alive rival already has 2+ cities (gate that
+/// filters early-game equal-start ties where everyone has 1 city), AND
+/// (b) `me`'s total population is the minimum among all alive players
+/// (including `me`). Used by `decide_military_action` to gate sole-city
+/// turtling so only the genuine underdog goes defensive — without this gate
+/// every clan at 1 city turtles and the whole map stalls at tier_peak=1.
+fn compute_is_trailing(state: &TacticalState, me: &TacticalPlayerState) -> bool {
+    let total_pop = |p: &TacticalPlayerState| -> u32 {
+        p.cities.iter().map(|c| c.population).sum()
+    };
+    let my_pop = total_pop(me);
+    let mut anyone_multi_city = false;
+    let mut anyone_below_me = false;
+    for (i, p) in state.players.iter().enumerate() {
+        if i == me.index as usize || p.cities.is_empty() {
+            continue;
+        }
+        if p.cities.len() >= 2 {
+            anyone_multi_city = true;
+        }
+        if total_pop(p) < my_pop {
+            anyone_below_me = true;
+        }
+    }
+    anyone_multi_city && !anyone_below_me
+}
+
 fn count_own_military_at(me: &TacticalPlayerState, pos: (i32, i32)) -> usize {
     me.units
         .iter()
@@ -500,6 +530,7 @@ fn decide_military_action(
     weights: &ScoringWeights,
     _rng: &mut XorShift64,
     difficulty_threshold_mult: f32,
+    is_trailing: bool,
 ) -> Option<Action> {
     let hp_frac = unit.hp as f32 / unit.hp_max.max(1) as f32;
     let nearest_enemy = nearest_enemy_unit(unit.hex, enemy_units);
@@ -560,14 +591,16 @@ fn decide_military_action(
     } else {
         retreat_hp_fraction
     };
-    // p1-29d — when we have exactly one city AND the enemy field outnumbers
-    // our own, units stay much more cautious: retreat at full HP and never
-    // press forward into a likely losing engagement. Composes additively on
-    // top of the personality-axis-driven base so aggressive clans still
-    // engage when they have parity, but a trailing AI no longer feeds the
-    // leader's kill count before being able to mass a real defense force.
+    // p1-29d — sole-city turtling for the *trailing* AI only (`is_trailing`
+    // is computed by `decide_movement` from full TacticalState: lowest total
+    // population AND at least one rival already multi-city). Without the
+    // is_trailing gate, every clan at 1 city in early game qualifies and the
+    // entire map turtles → no one expands → tier_peak=1 universal
+    // (huge-map batch 20260516_191254). When the gate fires, retreat HP
+    // threshold goes up by +0.30 (cap 0.90) so the trailing player stops
+    // feeding the leader's kill count before massing a real defense.
     let sole_city_threatened =
-        me.cities.len() == 1 && enemy_mil_count >= own_mil_count.max(1);
+        is_trailing && me.cities.len() == 1 && enemy_mil_count >= own_mil_count.max(1);
     let base_retreat_hp = if sole_city_threatened {
         (base_retreat_hp + 0.30).min(0.90)
     } else {