test: a watch's weight by age, and each contrast test under its reason

`draw.recency` weighs every watch of both affinities: an undated watch
and a future one now have a test (deleting either branch fails it). The
over-the-picture contrast test moves below the form-on-chrome test, so
each sits under the comment that explains it.

backend/tests/test_surprise_affinity.py

@@ -42,6 +42,20 @@ def test_a_short_side_is_topped_up_from_the_other():
     assert draw.blend(["a", "b", "c"], [], 0, 3) == ["a", "b", "c"]
 
 
+def test_a_watch_weighs_by_its_age_an_undated_one_two_half_lives_and_a_future_one_now():
+    # Both affinities weigh every watch through this one rule; an old import has no date, and a
+    # clock ahead of the server's must not weigh more than a watch made now.
+    assert draw.recency(NOW, NOW, 30) == 1.0
+    assert draw.recency(NOW - timedelta(days=30), NOW, 30) == 0.5
+    assert draw.recency(None, NOW, 30) == 0.25
+    assert draw.recency(NOW + timedelta(days=5), NOW, 30) == 1.0
+
+
+def test_scores_are_scaled_to_the_strongest_and_nothing_scored_is_empty():
+    assert draw.scaled({"a": 4.0, "b": 1.0}) == {"a": 1.0, "b": 0.25}
+    assert draw.scaled({}) == {}
+
+
 # ── YouTube: the channels the user watches ───────────────────────────────────────────────────────
 
 

frontend/src/lib/theme.contract.test.ts

@@ -59,17 +59,6 @@ describe("the media colours", () => {
   // accent for what must be seen: a light theme's are dark (#b91c1c 2.8:1, Slate's accent 2.8:1 on
   // the chrome). Its error edge and text take the media red, and every focus ring in it — the base
   // ring and a field's focus edge both read --focus — the media amber. [WCAG 1.4.11, 1.4.3]
-  // Text over the picture sits on a black panel the frame shows through: its worst ground is a white
-  // frame. The error line on it is the media red (a light theme's danger was 1.6:1 there). [WCAG 1.4.3]
-  it("an error over the picture holds 4.5:1 on its panel over a white frame", () => {
-    const player = readFileSync(new URL("../modules/plex/PlexPlayer.tsx", import.meta.url), "utf8");
-    const over = /const OVER_VIDEO =\s*"([^"]+)"/.exec(player)?.[1] ?? "";
-    expect(over.split(" ")).toContain("media-scope"); // --danger is the media red under it
-    const alpha = Number(/\bbg-black\/(\d+)\b/.exec(over)?.[1]) / 100;
-    const ground = mixHex("#000000", "#ffffff", alpha);
-    expect(hexContrast(media()["media-danger"]!, ground)).toBeGreaterThanOrEqual(4.5);
-    expect(hexContrast("#ffffff", ground)).toBeGreaterThanOrEqual(4.5);
-  });
   it("a form on the player chrome says an error and a focus in colours that hold there", () => {
     expect(hexContrast(media()["media-danger"]!, media()["media-chrome"]!)).toBeGreaterThanOrEqual(
       4.5,
@@ -91,6 +80,17 @@ describe("the media colours", () => {
     expect(field).toMatch(/\bfocus-within:ring-focus\b/);
     expect(field).not.toMatch(/\bfocus-within:(border|ring)-accent\b/);
   });
+  // Text over the picture sits on a black panel the frame shows through: its worst ground is a white
+  // frame. The error line on it is the media red (a light theme's danger was 1.6:1 there). [WCAG 1.4.3]
+  it("an error over the picture holds 4.5:1 on its panel over a white frame", () => {
+    const player = readFileSync(new URL("../modules/plex/PlexPlayer.tsx", import.meta.url), "utf8");
+    const over = /const OVER_VIDEO =\s*"([^"]+)"/.exec(player)?.[1] ?? "";
+    expect(over.split(" ")).toContain("media-scope"); // --danger is the media red under it
+    const alpha = Number(/\bbg-black\/(\d+)\b/.exec(over)?.[1]) / 100;
+    const ground = mixHex("#000000", "#ffffff", alpha);
+    expect(hexContrast(media()["media-danger"]!, ground)).toBeGreaterThanOrEqual(4.5);
+    expect(hexContrast("#ffffff", ground)).toBeGreaterThanOrEqual(4.5);
+  });
 
   it("every surface painted the player chrome names its marks: the focus ring and what is chosen", () => {
     const block = (name: string) => {