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/Tests/EditMode/EnemyAIMathMC2Tests.cs

@@ -0,0 +1,49 @@
+using NUnit.Framework;
+using ProjectM.Simulation;
+using Unity.Mathematics;
+
+namespace ProjectM.Tests
+{
+    /// <summary>
+    /// MC-2 pure-math tests for the new EnemyAIMath helpers: BandVelocity (Spitter range-band keep-distance) and
+    /// ClusterOffset (swarmer pack placement). No ECS world.
+    /// </summary>
+    public class EnemyAIMathMC2Tests
+    {
+        [Test]
+        public void BandVelocity_AdvancesWhenTooFar()
+        {
+            var v = EnemyAIMath.BandVelocity(new float3(0, 1, 0), new float3(20, 1, 0), 5f, 9f, 1.5f);
+            Assert.Greater(v.x, 0.1f, "too far -> moves toward the target");
+            Assert.AreEqual(0f, v.y, 1e-5f, "planar");
+        }
+
+        [Test]
+        public void BandVelocity_RetreatsWhenTooClose()
+        {
+            var v = EnemyAIMath.BandVelocity(new float3(0, 1, 0), new float3(3, 1, 0), 5f, 9f, 1.5f);
+            Assert.Less(v.x, -0.1f, "too close -> backs away from the target");
+        }
+
+        [Test]
+        public void BandVelocity_HoldsInBand()
+        {
+            var v = EnemyAIMath.BandVelocity(new float3(0, 1, 0), new float3(9, 1, 0), 5f, 9f, 1.5f);
+            Assert.AreEqual(0f, math.length(v), 1e-4f, "inside the dead-zone band -> hold and fire");
+        }
+
+        [Test]
+        public void ClusterOffset_SingleAtCentre_PackSpread()
+        {
+            var c = new float3(100, 1, 5);
+            var single = EnemyAIMath.ClusterOffset(c, 0, 1, 2.5f);
+            Assert.AreEqual(c.x, single.x, 1e-5f, "a lone swarmer spawns at the pack centre");
+            Assert.AreEqual(c.z, single.z, 1e-5f);
+            var a = EnemyAIMath.ClusterOffset(c, 0, 4, 2.5f);
+            var b = EnemyAIMath.ClusterOffset(c, 1, 4, 2.5f);
+            Assert.Greater(math.distance(a, b), 0.01f, "pack members get distinct offsets");
+            var again = EnemyAIMath.ClusterOffset(c, 2, 4, 2.5f);
+            Assert.AreEqual(0f, math.distance(again, EnemyAIMath.ClusterOffset(c, 2, 4, 2.5f)), 1e-5f, "deterministic");
+        }
+    }
+}