Slice Combat Depth (MC-2): enemy-variety server spine — Spitter, Swarmer, 4-type mix (DR-041)

Adds the server-authoritative mechanics for three new enemy archetypes on top of
the Grunt/Charger base, plus the weighted wave-composition that introduces them:

- Spitter: a ranged Husk variant (SpitterState) that holds a preferred range-band
  (advance/retreat/hold via EnemyAIMath.BandVelocity) and fires a telegraphed,
  dodgeable EnemyProjectile. New server EnemyProjectileMoveSystem (integrate +
  store LastStep) + EnemyProjectileDamageSystem (region-filtered swept hit-test
  rebuilt from LastStep — DR-018 anti-tunnelling; players use HitRadius, structures
  a const radius; at-most-once destroy). Concurrent-spit soft cap, soft-fail retry.
- Swarmer: marker tag + deterministic cluster spawn (1 slot = 1 pack;
  EnemyAIMath.ClusterOffset), MaxAlive counts ENTITIES so a pack defers if it
  won't fit.
- 4-type weighted mix: MixBands -> ZoneEnemyMath.WaveSlots/KindForSlot/
  PackSizeForSlot drives both the expedition director and (fork-4a) the base siege,
  with a mandatory MaxAlive cap. Legacy WaveSize/IsChargerSlot kept + parity-tested.
- Discriminator stays component-presence (no enum in Bursted systems): query-
  partition guards keep each enemy moved by exactly one EnemyAISystem pass
  (sole-Position-writer). EnemyTelegraph.IsCharger -> Kind byte for the client cue.

New authoring (Spitter/Swarmer/EnemyProjectile) + expanded director authorings with
tunable mix/cluster defaults. 13 new EditMode tests (mix composition + legacy parity,
band/cluster math, projectile move + cross-region + swept anti-tunnelling regressions);
full suite green before commit.

Dormant until the prefab/subscene wiring lands (next): the new systems guard on
TryGetSingleton/RequireForUpdate, so with no prefabs wired the new types stay inert.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/_Project/Scripts/Simulation/Combat/EnemyAIMath.cs

@@ -126,5 +126,42 @@ namespace ProjectM.Simulation
                 if (sq < bestSq) { bestSq = sq; index = i; isStructure = true; }
             }
         }
+
+        /// <summary>
+        /// MC-2 Spitter range-band velocity (planar XZ): ADVANCE toward <paramref name="to"/> at
+        /// <paramref name="speed"/> when farther than <paramref name="preferred"/> + <paramref name="tolerance"/>,
+        /// RETREAT directly away when closer than <paramref name="preferred"/> - <paramref name="tolerance"/>, and
+        /// HOLD (zero) inside the dead-zone band. Y forced to 0. Returns zero when the points coincide. Pure /
+        /// Burst-safe / EditMode-testable; keeps the Spitter at its firing distance instead of closing to melee.
+        /// </summary>
+        public static float3 BandVelocity(float3 from, float3 to, float speed, float preferred, float tolerance)
+        {
+            float3 d = to - from;
+            d.y = 0f;
+            float distSq = math.lengthsq(d);
+            if (distSq < 1e-8f)
+                return float3.zero;
+            float dist = math.sqrt(distSq);
+            float3 dir = d / dist;
+            float tol = math.max(0f, tolerance);
+            if (dist > preferred + tol)
+                return dir * speed;          // too far -> close in
+            if (dist < preferred - tol)
+                return -dir * speed;         // too close -> back off
+            return float3.zero;              // in band -> hold and fire
+        }
+
+        /// <summary>
+        /// Deterministic tight-cluster offset for swarmer <paramref name="index"/> of a pack of
+        /// <paramref name="packSize"/> around <paramref name="center"/> at <paramref name="tightRadius"/> (reuses the
+        /// <see cref="RingPosition"/> even-angle math at a small radius). A single swarmer (packSize&lt;=1) spawns at
+        /// the centre. Stable per index so a replayed pack lands identically. Pure.
+        /// </summary>
+        public static float3 ClusterOffset(float3 center, int index, int packSize, float tightRadius)
+        {
+            if (packSize <= 1)
+                return center;
+            return RingPosition(center, index, packSize, tightRadius);
+        }
     }
 }