feat(render): fade railroad overlay near min zoom

Replace the hard zoom cutoff in RailroadPass with a linear alpha fade
controlled by a new `railFadeRange` setting. Rails (and bridge pixels)
ramp from invisible at `railMinZoom - railFadeRange` to fully opaque at
`railMinZoom`, instead of popping in. Adds a uRailFade shader uniform
and a debug slider.

src/client/render/gl/RenderSettings.ts

@@ -81,6 +81,7 @@ export interface RenderSettings {
   };
   railroad: {
     railMinZoom: number;
+    railFadeRange: number;
     railDetailZoom: number;
     railAlpha: number;
   };

src/client/render/gl/debug/Layout.ts

@@ -162,6 +162,15 @@ export function buildTree(s: RenderSettings, d: RenderSettings): DebugNode[] {
 
       folder("Railroad", [
         slider(s.railroad, "railMinZoom", d.railroad, 0, 10, 0.1, "Min Zoom"),
+        slider(
+          s.railroad,
+          "railFadeRange",
+          d.railroad,
+          0,
+          5,
+          0.1,
+          "Fade Range",
+        ),
         slider(
           s.railroad,
           "railDetailZoom",

src/client/render/gl/passes/RailroadPass.ts

@@ -98,6 +98,7 @@ export class RailroadPass {
   private uZoom: WebGLUniformLocation;
   private uRailDetailZoom: WebGLUniformLocation;
   private uRailAlpha: WebGLUniformLocation;
+  private uRailFade: WebGLUniformLocation;
   private uGhostOwnerID: WebGLUniformLocation;
 
   private mapW: number;
@@ -145,6 +146,7 @@ export class RailroadPass {
       "uRailDetailZoom",
     )!;
     this.uRailAlpha = gl.getUniformLocation(this.program, "uRailAlpha")!;
+    this.uRailFade = gl.getUniformLocation(this.program, "uRailFade")!;
     this.uGhostOwnerID = gl.getUniformLocation(this.program, "uGhostOwnerID")!;
 
     // Texture unit bindings + ghost defaults
@@ -265,8 +267,16 @@ export class RailroadPass {
     const gl = this.gl;
     const rs = this.settings.railroad;
 
-    // Skip entirely when below minimum zoom
-    if (zoom < rs.railMinZoom) return;
+    // Fade out as zoom drops below railMinZoom; fully invisible at railMinZoom - railFadeRange
+    const fadeRange = Math.max(rs.railFadeRange, 0);
+    const fadeStart = rs.railMinZoom - fadeRange;
+    const fade =
+      fadeRange <= 0
+        ? zoom >= rs.railMinZoom
+          ? 1
+          : 0
+        : Math.min(1, Math.max(0, (zoom - fadeStart) / fadeRange));
+    if (fade <= 0) return;
 
     // Flush CPU railroad state → GPU
     if (this.railroadDirty) {
@@ -310,6 +320,7 @@ export class RailroadPass {
     gl.uniform1f(this.uZoom, zoom);
     gl.uniform1f(this.uRailDetailZoom, rs.railDetailZoom);
     gl.uniform1f(this.uRailAlpha, rs.railAlpha);
+    gl.uniform1f(this.uRailFade, fade);
     gl.uniform1f(this.uGhostOwnerID, this.ghostOwnerID);
 
     // Bind textures: 0=railroad, 1=tile, 2=palette, 3=terrain, 4=ghostRail

src/client/render/gl/render-settings.json

@@ -79,6 +79,7 @@
   },
   "railroad": {
     "railMinZoom": 4,
+    "railFadeRange": 2,
     "railDetailZoom": 6,
     "railAlpha": 1
   },

src/client/render/gl/shaders/railroad/railroad.frag.glsl

@@ -12,6 +12,7 @@ uniform vec2 uMapSize;
 uniform float uZoom;
 uniform float uRailDetailZoom;
 uniform float uRailAlpha;
+uniform float uRailFade;             // Zoom-based fade multiplier (0..1)
 uniform float uGhostOwnerID;         // Player smallID for ghost rail color
 
 in vec2 vWorldPos;
@@ -139,17 +140,17 @@ void main() {
     // Overlapping railroad highlight — green tint
     if (highlighted) railColor = vec3(0.2, 0.85, 0.3);
     if (hitBridge) {
-      fragColor = vec4(mix(bridgeColor, railColor, railAlpha), 1.0);
+      fragColor = vec4(mix(bridgeColor, railColor, railAlpha), uRailFade);
     } else {
-      fragColor = vec4(railColor, railAlpha);
+      fragColor = vec4(railColor, railAlpha * uRailFade);
     }
   } else if (hitGhost) {
     float ghostAlpha = uRailAlpha * ghostCov * 0.5;
     vec3 ghostColor = uGhostOwnerID > 0.0
       ? texture(uPalette, vec2((uGhostOwnerID + 0.5) / float(PALETTE_SIZE), 0.75)).rgb
       : vec3(0.75);
-    fragColor = vec4(ghostColor, ghostAlpha);
+    fragColor = vec4(ghostColor, ghostAlpha * uRailFade);
   } else {
-    fragColor = vec4(bridgeColor, 1.0);
+    fragColor = vec4(bridgeColor, uRailFade);
   }
 }