Add Ground Zero water source visual gate

Docs/Terrain/GroundZeroWaterSourceVisualPass.md

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+# Ground Zero Water Source Visual Pass
+
+The 0.1.O investor visual pass gives the MVP freshwater source an explicit
+readable presentation instead of relying on a flat placeholder or a generic
+pickup shape.
+
+## Required Presentation
+
+- The native `AAgrarianWaterSource` actor exposes a visible water surface proxy
+  using `/Game/Agrarian/Materials/M_AGR_GZ_FreshWater`.
+- The same actor exposes stone bank/edge treatment using
+  `/Game/Agrarian/Materials/M_AGR_GZ_Stone_Sandstone`.
+- A small collect marker remains on the actor so the source reads as gameplay
+  relevant without requiring tutorial text.
+- The Ground Zero map keeps the freshwater source at the drainage-candidate
+  location near `(-7200, 10400)` so it remains reachable but not on top of the
+  safe spawn point.
+- Nearby environmental dressing includes a visible water-surface plane,
+  stone-bank piece, and reed/grass pieces so the source reads as a small
+  collectable freshwater feature rather than an isolated debug primitive.
+
+## Validation
+
+`Scripts/verify_ground_zero_water_source_visual_pass.py` loads the Ground Zero
+map and checks:
+
+- exactly one `AGR_GZ_FreshWaterSource_01` actor exists;
+- the actor is an `AAgrarianWaterSource`;
+- the native `Mesh`, `WaterSurfaceProxy`, `StoneBankProxy`, and
+  `CollectMarkerProxy` components exist;
+- the water surface uses the freshwater material and is scaled large enough to
+  be readable;
+- the bank/root components use the sandstone material;
+- the actor remains at the intended drainage-candidate placement;
+- the nearby water surface, water bank, and reed variation actors are present
+  within a short radius of the source.
+
+This pass is still MVP proxy art. Final water rendering should replace the
+proxy meshes with production water material, bank geometry, wet edge blending,
+sound, and collection animation without changing the gameplay interaction
+contract.