Improve railroad visibility: own-rail contrast color and thickness setting

Local-player rails previously rendered in the white focused-border color
from the palette, making them hard to see on light territory. Rails now
use a dedicated local rail color: white normally, flipped to black when
the territory backdrop is too light for white to read against (patterns
average their primary/secondary brightness).

Also add a railThickness render setting (0.5-3, default 1), exposed in
the Graphics Settings modal and the debug GUI, and persisted via
GraphicsOverrides. In the medium-zoom LOD, rails are now drawn as
screen-space anti-aliased lines around each tile's rail centerline,
accumulated from the 3x3 neighborhood so thick lines spill cleanly into
neighboring tiles; detailed mode scales its sub-grid band widths.

- PlayerView: compute railColor() (white/black by backdrop brightness)
- RailroadPass/shader: uLocalPlayerID, uLocalRailColor, uRailThickness
- render-settings.json, RenderSettings, GraphicsOverrides,
  RenderOverrides: new railroad.railThickness knob
- GraphicsSettingsModal: "Train track thickness" slider (+ en.json keys)
- tests: schema + apply coverage for railroad overrides

tests/GraphicsOverrides.test.ts

@@ -54,6 +54,18 @@ describe("GraphicsOverridesSchema", () => {
     }
   });
 
+  test("accepts partial railroad overrides", () => {
+    const cases = [
+      { railroad: {} },
+      { railroad: { railMinZoom: 2 } },
+      { railroad: { railThickness: 1.5 } },
+      { railroad: { railMinZoom: 0, railThickness: 3 } },
+    ];
+    for (const c of cases) {
+      expect(GraphicsOverridesSchema.safeParse(c).success).toBe(true);
+    }
+  });
+
   test("rejects wrong field types", () => {
     expect(
       GraphicsOverridesSchema.safeParse({ name: { nameScaleFactor: "big" } })
@@ -72,6 +84,16 @@ describe("GraphicsOverridesSchema", () => {
         mapOverlay: { territorySaturation: "full" },
       }).success,
     ).toBe(false);
+    expect(
+      GraphicsOverridesSchema.safeParse({
+        railroad: { railMinZoom: "far" },
+      }).success,
+    ).toBe(false);
+    expect(
+      GraphicsOverridesSchema.safeParse({
+        railroad: { railThickness: "wide" },
+      }).success,
+    ).toBe(false);
   });
 });
 
@@ -214,6 +236,31 @@ describe("applyGraphicsOverrides", () => {
     expect(mo.territoryDefenseDarken).toBe(defaults.territoryDefenseDarken);
   });
 
+  test("applies railMinZoom override (including 0)", () => {
+    expect(gen({ railroad: { railMinZoom: 7 } }).railroad.railMinZoom).toBe(7);
+    expect(gen({ railroad: { railMinZoom: 0 } }).railroad.railMinZoom).toBe(0);
+  });
+
+  test("applies railThickness override (including values below 1)", () => {
+    expect(
+      gen({ railroad: { railThickness: 2.5 } }).railroad.railThickness,
+    ).toBe(2.5);
+    expect(
+      gen({ railroad: { railThickness: 0.5 } }).railroad.railThickness,
+    ).toBe(0.5);
+  });
+
+  test("railroad override leaves other railroad fields at defaults", () => {
+    const defaults = createRenderSettings().railroad;
+    const r = gen({ railroad: { railThickness: 2 } }).railroad;
+    expect(r.railMinZoom).toBe(defaults.railMinZoom);
+    expect(r.railFadeRange).toBe(defaults.railFadeRange);
+    expect(r.railDetailZoom).toBe(defaults.railDetailZoom);
+    expect(r.railAlpha).toBe(defaults.railAlpha);
+    const z = gen({ railroad: { railMinZoom: 1 } }).railroad;
+    expect(z.railThickness).toBe(defaults.railThickness);
+  });
+
   test("classicIcons + name overrides compose independently", () => {
     const s = gen({
       name: { darkNames: true, nameScaleFactor: 0.9 },