Run 12: knowledge system, mortality, and the North's generational soul

SPEC §5 built: three skill domains (forage/woodcraft/masonry) that pay
off and grow by practice; proximity diffusion soul-calibrated by
absorption x teaching-drive; and mortality - the piece that makes
knowledge generational. Elders die (~150-230 days), everything unshared
dies with them, a youth comes of age near knowing nothing. North soul
(absorption 1.6, teaching 0.9) vs South (1.0, 0.3); East teaches no one.

Two findings: (1) headless soaks were contaminated by orphaned child
processes writing later runs' CSVs - fixed with StopAfterDays clean
self-terminate; (2) passive diffusion barely helped - the North's real
culture is the young seeking elders on purpose, so added deliberate
teacher-seeking and youths now cradle beside the wisest elder. North
now leads knowledge at every checkpoint, but a stable town approaches
the ceiling regardless, so teaching is a recovery advantage not an
equilibrium one. The crisis-recovery experiment is the real test, noted
in FINDINGS as next.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/NPC.cs

@@ -33,6 +33,13 @@ public class Npc
     public float Health = 100f;                  // forced rest below 30
     public float Nourishment = 1f;               // 1 = fed; drains daily, eats from pack
 
+    // --- Mortality: knowledge is generational only if generations end ----
+    public float AgeDays;
+    public float LifeExpectancyDays = 180f;
+
+    // --- Knowledge (SPEC §5) ----------------------------------------------
+    public Knowledge Know = new();
+
     // --- State -----------------------------------------------------------
     public NpcState State = NpcState.Gathering;
 
@@ -78,6 +85,33 @@ public class Npc
     /// who they are becoming. Indexed by (int)NpcState.</summary>
     public int[] StateTicksToday = new int[5];
 
+    /// <summary>
+    /// Learn from another by proximity (SPEC §5). Rate scales with your own
+    /// absorption and their drive to teach. Returns true if anything passed.
+    /// </summary>
+    private bool LearnFrom(Npc other)
+    {
+        bool learned = false;
+        float rate = 0.0016f * Soul.Absorption * (1f + other.Soul.TeachingDrive * 2.5f);
+        for (int d = 0; d < Knowledge.DomainCount; d++)
+        {
+            float gap = other.Know.Skill[d] - Know.Skill[d];
+            if (gap > 0.03f)
+            {
+                Know.Gain((KnowledgeDomain)d, gap * rate);
+                learned = true;
+            }
+        }
+        return learned;
+    }
+
+    /// <summary>Clear live references to someone who has died.</summary>
+    public void ForgetPerson(Npc dead)
+    {
+        if (_targetNpc == dead) _targetNpc = null;
+        if (_giftSource == dead) _giftSource = null;
+    }
+
     // --- Diagnostics (read-only views for the stats logger) ---------------
     public MaterialKind CurrentDemand(GameManager gm) => PickGatherTarget(gm);
     public int RoughNights => _unshelteredRests;
@@ -426,8 +460,12 @@ public class Npc
         else
         {
             Activity = $"harvesting {_targetNode.Kind.ToString().ToLower()}";
-            float taken = world.Harvest(_targetNode, requested: 2f, this);
+            // Skill pays: a practiced hand takes more per reach — and the
+            // work itself is the teacher.
+            var domain = Knowledge.DomainFor(_targetNode.Kind);
+            float taken = world.Harvest(_targetNode, requested: 2f * (1f + Know[domain]), this);
             if (taken <= 0f) { _targetNode = null; return; }  // node at its floor — re-seek
+            Know.Gain(domain, 0.0006f);
             Inventory.TryGetValue(_targetNode.Kind, out float have);
             Inventory[_targetNode.Kind] = have + taken;
 
@@ -645,9 +683,42 @@ public class Npc
 
     private void TickSocializing(GameManager gm)
     {
-        // Drift toward the nearest neighbor — community forms where feet do.
+        // The young seek a teacher. A soul that still has much to learn and
+        // the drive to learn it looks for the wisest elder in town rather
+        // than drifting to whoever's closest. This is the North's culture in
+        // motion: the young sitting at the feet of the old, on purpose.
         Npc? nearest = null;
         float bestDist = float.MaxValue;
+
+        if (Soul.Absorption > 1.2f && Know.Total < Know.Capacity * 0.6f)
+        {
+            Npc? teacher = null;
+            float bestWorth = 0.15f;   // must actually know more than us
+            foreach (var other in gm.Npcs)
+            {
+                if (other == this || other.HomeTown != HomeTown) continue;
+                float worth = (other.Know.Total - Know.Total) * (1f + other.Soul.TeachingDrive);
+                if (worth > bestWorth) { bestWorth = worth; teacher = other; }
+            }
+            if (teacher != null)
+            {
+                if (Position.DistanceTo(teacher.Position) > 3f)
+                {
+                    Activity = $"seeking out {teacher.Name} to learn";
+                    MoveToward(teacher.Position);
+                }
+                else
+                {
+                    LearnFrom(teacher);
+                    Activity = $"learning from {teacher.Name}";
+                }
+                if (!gm.IsNight && _stateTimer > 15f) Enter(NpcState.Gathering);
+                return;
+            }
+        }
+
+        // Otherwise: drift toward the nearest neighbor — community forms
+        // where feet do.
         foreach (var other in gm.Npcs)
         {
             if (other == this) continue;
@@ -659,9 +730,15 @@ public class Npc
             Activity = $"walking over to {nearest.Name}";
             MoveToward(nearest.Position);
         }
+        else if (nearest != null)
+        {
+            Activity = LearnFrom(nearest)
+                ? $"learning from {nearest.Name}"
+                : $"chatting with {nearest.Name}";
+        }
         else
         {
-            Activity = nearest != null ? $"chatting with {nearest.Name}" : "idling";
+            Activity = "idling";
         }
 
         // TODO(Week 3): bonds + Generosity-weighted spontaneous help.