test(labs): assert the outlier's distance from the edge, not one string

Follow-up to #860, promised in review rather than asked of the
contributor. `not.toContain("left: 4%")" passes for a marker at 4.02%,
which is the same defect one rounding away, so the test now reads the
marker position and asserts it sits inside the padding at both ends.

EDGE_PADDING_PERCENT moves to module scope and is exported, so the
assertion uses the number the component clamps with instead of a copy
that can drift. It was being reallocated on every render besides.

Verified by mutation: dropping the outlier expansion puts the marker at
exactly 4 and turns this test red.

src/components/targets/__tests__/range-bar.test.tsx

@@ -2,7 +2,7 @@ import { describe, expect, it } from "vitest";
 import { renderToStaticMarkup } from "react-dom/server";
 
 import { I18nProvider } from "@/lib/i18n/context";
-import { RangeBar } from "../range-bar";
+import { EDGE_PADDING_PERCENT, RangeBar } from "../range-bar";
 
 /**
  * v1.4.25 W3e — `<RangeBar>` was extracted from the inline 790-line
@@ -68,7 +68,18 @@ describe("<RangeBar>", () => {
       tone: "lab",
     });
 
-    expect(html).not.toContain("left: 4%");
+    // Assert the distance, not one unlucky string. `not.toContain("left: 4%")`
+    // passes for a marker at 4.02%, which is the same defect one rounding away.
+    // The marker is the fourth `left:` in the markup — the three before it are
+    // the bar slot and the band edges.
+    const positions = [...html.matchAll(/left:\s*([0-9.]+)%/g)].map((m) =>
+      Number(m[1]),
+    );
+    const marker = positions[3];
+
+    expect(marker).toBeGreaterThan(EDGE_PADDING_PERCENT);
+    // And not flung to the other end by an over-eager expansion.
+    expect(marker).toBeLessThan(100 - EDGE_PADDING_PERCENT);
   });
 
   it("makes the marker focusable with a text alternative for the value + range (2026-07-17 a11y audit M1)", () => {

src/components/targets/range-bar.tsx

@@ -30,6 +30,14 @@ export interface RangeBarProps {
   maxLabel?: string | null;
 }
 
+/**
+ * How far from either end the value marker is allowed to sit, in percent.
+ *
+ * Module scope rather than the render body so the test can assert against the
+ * same number the component clamps with, instead of a copy that can drift.
+ */
+export const EDGE_PADDING_PERCENT = 4;
+
 export function RangeBar({
   value,
   min,
@@ -83,7 +91,6 @@ export function RangeBar({
   const visualSpan = visualMax - visualMin;
   const clampedValue = Math.max(visualMin, Math.min(visualMax, value));
   const rawPosition = ((clampedValue - visualMin) / visualSpan) * 100;
-  const EDGE_PADDING_PERCENT = 4;
   const position = Math.max(
     EDGE_PADDING_PERCENT,
     Math.min(100 - EDGE_PADDING_PERCENT, rawPosition),