Hazard: Blight geyser — permanent periodic both-sides AoE (Phase 1.5b bundle 3)

Review-first netcode slice (design review wf_900e9965-8f0 -> 11 folded;
post-impl wf_5c8299f8-299 clean). A PERMANENT periodic telegraphed AoE in
Blight-biome rooms only; one new [GhostField] uint NextEruptTick.

- Geyser component + GeyserEruptSystem (server): inverted invalid-tick guard
  (a baked-0 tick must NEVER erupt -> lazy-stamps born-correct instead of the
  party-wiping per-tick barrage), inline both-sides gather (SourceNetworkId=-1),
  reschedule = now + period (never +=), never destroyed.
- GeyserTelegraphSystem (client): absolute-tick growing warning disc +
  erupt burst on the >0->=<0 crossing, latched per NextEruptTick + arm-guard
  (never edge-detects the replicated field -> no phantom on 0->stamp /
  relevancy re-entry). Reuses ScorchDecalSystem + DynamicLightSystem.
- Seeded in RoomFieldSystem (Blight-gated on plan.Biome, born-correct staggered
  stamp from live ServerTick), GeyserFieldSpawner + authoring wired into the
  Gameplay subscene. Geyser.prefab duplicated from ResourceNode (no collider).
- BuildDisc promoted to FeedbackFx (shared with the barrel fuse ring).
- 474/474 EditMode incl. the unstamped-storm regression; live no-storm end-to-end.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Assets/_Project/Scripts/Client/Presentation/FeedbackFx.cs

@@ -180,5 +180,29 @@ namespace ProjectM.Client
             m.RecalculateBounds();
             return m;
         }
+
+        // Unit-radius upward-facing disc fan on the XZ plane (centre vertex + rim) for ground telegraph / blast-radius
+        // rings — scale x/z to the radius. No vertex colours (white default) so the material or a per-renderer _Color
+        // MPB fully tints + fades it. Promoted from WorldFeedbackSystem so the barrel fuse ring + the geyser telegraph
+        // share ONE primitive (review M6).
+        public static Mesh BuildDisc(int segments)
+        {
+            if (segments < 6) segments = 6;
+            var m = new Mesh { name = "Disc" };
+            var v = new Vector3[segments + 1];
+            var tris = new int[segments * 3];
+            v[0] = Vector3.zero;
+            for (int i = 0; i < segments; i++)
+            {
+                float a = i / (float)segments * Mathf.PI * 2f;
+                v[i + 1] = new Vector3(Mathf.Cos(a), 0f, Mathf.Sin(a));
+                int n = (i + 1) % segments;
+                tris[i * 3] = 0; tris[i * 3 + 1] = n + 1; tris[i * 3 + 2] = i + 1;
+            }
+            m.vertices = v;
+            m.triangles = tris;
+            m.RecalculateBounds();
+            return m;
+        }
     }
 }