feat(editor): river/path carver tool for terrain channels

- River Carver in Sculpt panel: carves a channel between two points
  with configurable width and depth
- Set Start at cursor, then Set End + Carve to create the channel
- Smooth quadratic falloff at edges for natural riverbank shape
- Works by projecting each terrain vertex onto the line segment and
  lowering height based on distance from center
- Auto-stitches chunk edges after carving
- Pair with Water mode to fill the carved channel with liquid

tools/editor/editor_ui.cpp

@@ -459,6 +459,31 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
         }
 
         ImGui::Separator();
+        if (ImGui::CollapsingHeader("River / Path Carver")) {
+            static glm::vec3 riverStart{0}, riverEnd{0};
+            static float riverWidth = 8.0f, riverDepth = 5.0f;
+            static bool riverStartSet = false;
+            ImGui::SliderFloat("Width##river", &riverWidth, 2.0f, 50.0f);
+            ImGui::SliderFloat("Depth##river", &riverDepth, 1.0f, 30.0f);
+            auto& brush4 = app.getTerrainEditor().brush();
+            if (ImGui::Button("Set Start##river", ImVec2(120, 0)) && brush4.isActive()) {
+                riverStart = brush4.getPosition();
+                riverStartSet = true;
+                app.showToast("River start set");
+            }
+            ImGui::SameLine();
+            if (ImGui::Button("Set End + Carve##river", ImVec2(140, 0)) && brush4.isActive() && riverStartSet) {
+                riverEnd = brush4.getPosition();
+                app.getTerrainEditor().carveRiver(riverStart, riverEnd, riverWidth, riverDepth);
+                app.showToast("River carved");
+                riverStartSet = false;
+            }
+            if (riverStartSet)
+                ImGui::TextColored(ImVec4(0.5f, 0.9f, 0.5f, 1), "Start set — click end point");
+            else
+                ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1), "Set start then end to carve");
+        }
+
         if (ImGui::CollapsingHeader("Mirror Terrain")) {
             if (ImGui::Button("Mirror X (Left<>Right)", ImVec2(-1, 0))) {
                 app.getTerrainEditor().mirrorX();

tools/editor/terrain_editor.cpp

@@ -712,6 +712,46 @@ void TerrainEditor::mirrorY() {
     dirty_ = true;
 }
 
+void TerrainEditor::carveRiver(const glm::vec3& start, const glm::vec3& end,
+                                float width, float depth) {
+    if (!terrain_) return;
+    glm::vec2 lineStart(start.x, start.y);
+    glm::vec2 lineEnd(end.x, end.y);
+    glm::vec2 lineDir = glm::normalize(lineEnd - lineStart);
+    float lineLen = glm::length(lineEnd - lineStart);
+
+    for (int ci = 0; ci < 256; ci++) {
+        auto& chunk = terrain_->chunks[ci];
+        if (!chunk.hasHeightMap()) continue;
+        bool modified = false;
+
+        for (int v = 0; v < 145; v++) {
+            glm::vec3 pos = chunkVertexWorldPos(ci, v);
+            glm::vec2 p(pos.x, pos.y);
+
+            // Project point onto line segment
+            glm::vec2 toP = p - lineStart;
+            float t = glm::dot(toP, lineDir);
+            t = std::clamp(t, 0.0f, lineLen);
+            glm::vec2 closest = lineStart + lineDir * t;
+            float dist = glm::length(p - closest);
+
+            if (dist < width) {
+                float falloff = 1.0f - (dist / width);
+                falloff = falloff * falloff; // smooth edges
+                float carve = depth * falloff;
+                chunk.heightMap.heights[v] -= carve;
+                modified = true;
+            }
+        }
+        if (modified) {
+            stitchEdges(ci);
+            dirtyChunks_.push_back(ci);
+        }
+    }
+    dirty_ = true;
+}
+
 void TerrainEditor::copyStamp(const glm::vec3& center, float radius) {
     if (!terrain_) return;
     stampData_.clear();

tools/editor/terrain_editor.hpp

@@ -72,6 +72,9 @@ public:
     void mirrorX();
     void mirrorY();
 
+    // Carve a river/path between two points (lowers terrain along line)
+    void carveRiver(const glm::vec3& start, const glm::vec3& end, float width, float depth);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);