Slider: the set value to its step's decimals; the reading marked on the track and on the ring; mark geometry in the docs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/tests/widgets.spec.ts

@@ -184,7 +184,7 @@ test.beforeAll(async ({ browser }) => {
       config: {},
     },
     {
-      // No precision configured, so what it is worth is what decides how it is
+      // No precision configured, so its step's decimals decide how it is
       // written — including on the way there.
       id: "aim",
       type: "slider",
@@ -432,6 +432,8 @@ test("a slider writes what it sets against under its value", async ({
   await expect(page.getByTestId("slider-reading")).toHaveText(
     /^Level\s*80\.0\s*kW$/,
   )
+  // And placed on the track, as a mark beside the handle.
+  await expect(page.locator(".dui-slider-mark")).toBeVisible()
 })
 
 test("a reading is written to its own precision while it is moving", async ({