Add Ground Zero water source

Docs/Terrain/GroundZeroWaterSource.md

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+# Ground Zero Freshwater Source
+
+The Ground Zero map now has a first-pass gameplay freshwater source.
+
+## Placement
+
+- Actor label: `AGR_GZ_FreshWaterSource_01`
+- Blueprint: `/Game/Agrarian/Blueprints/World/BP_FreshWaterSource`
+- Placement intent: drainage-candidate area identified during the landform pass.
+
+The previous water/shoreline analysis found no ocean, lake, or confirmed stream
+inside the current 1 km tile. This actor is therefore a gameplay MVP freshwater
+source, not a confirmed hydrography feature.
+
+## Gameplay
+
+The water source is an interactable actor. On server-authoritative interaction,
+it restores thirst through `UAgrarianSurvivalComponent::AddWater`.
+
+- Restore amount: `45`
+- Interaction text: `Drink from Fresh Water Spring`
+
+## Follow-Up
+
+Later hydrography work should validate this drainage candidate against USGS NHD
+or equivalent data. If a confirmed stream or spring is available, replace this
+placeholder with the real watercourse location and geometry.