test(design): 钉住 D40 的彩度门槛 8（复审 r1 p2-3）

MIN_SEED_CHROMA 8.0 → 6.0 的变异原本全绿（那条断言比的是常量自己）。
新增两条用实测 chroma≈7.23 的灰蓝（#7E8489）：
- seed::tests::chroma_just_below_the_gate_is_rejected（门槛之上/之下夹住）
- resolve::tests::faint_colour_falls_back_to_the_selected_card（回落 + 落点是选中卡 + 方案逐值等于该卡）
顺带把 resolve 的单测拆到 resolve/tests.rs（400 行上限）

crates/design/src/palette/resolve/mod.rs

@@ -196,176 +196,4 @@ fn card_key(card: Card) -> CardKey {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::color::Color;
-
-    fn solid(color: Color, width: u32, height: u32) -> Vec<u8> {
-        let mut out = Vec::with_capacity((width * height * 4) as usize);
-        for _ in 0..(width * height) {
-            out.extend_from_slice(&[color.r, color.g, color.b, color.a]);
-        }
-        out
-    }
-
-    #[test]
-    fn level_one_wins_when_background_is_good() {
-        let pixels = solid(Color::rgb(0xC0, 0x39, 0x2B), 4, 4);
-        let (scheme, provenance) = resolve_source(
-            Some(ImageInput::pixels(&pixels, 4, 4)),
-            None,
-            Some(CardKey::Forest),
-            SchemeFlavor::Light,
-        );
-        assert_eq!(provenance.level, Level::BackgroundImage);
-        assert_eq!(provenance.background(), Outcome::Used);
-        assert_eq!(provenance.preset(), Outcome::Absent);
-        assert!(provenance.hint_key().is_none());
-        assert!(check(&scheme).is_clean());
-        assert!(provenance.seed.is_some());
-        // 取色方案的表面色温来自当前色卡（森林绿），不是默认卡。
-        assert_eq!(provenance.card, CardKey::Forest);
-    }
-
-    #[test]
-    fn gray_background_falls_back_to_wallpaper_with_reason() {
-        // 反证 1：灰白背景图 → 不许凭空上色，必须带着 low-chroma 原因回落。
-        let gray = solid(Color::rgb(0xF2, 0xF2, 0xF0), 8, 8);
-        let teal = solid(Color::rgb(0x00, 0x79, 0x6B), 8, 8);
-        let (_, provenance) = resolve_source(
-            Some(ImageInput::pixels(&gray, 8, 8)),
-            Some(ImageInput::pixels(&teal, 8, 8)),
-            Some(CardKey::Sky),
-            SchemeFlavor::Light,
-        );
-        assert_eq!(provenance.level, Level::Wallpaper);
-        match provenance.background() {
-            Outcome::Failed(Skip::LowChroma { chroma, .. }) => assert!(chroma < 8.0),
-            other => panic!("背景图应当是低彩度失败，实际 {other:?}"),
-        }
-        assert_eq!(
-            provenance.hint_key(),
-            Some("palette.hint.background.low-chroma")
-        );
-        assert!(provenance.did_fall_back());
-        assert_eq!(
-            provenance.reason_codes(),
-            vec![("background-image", "low-chroma")]
-        );
-    }
-
-    #[test]
-    fn undecodable_and_tiny_inputs_are_distinguished_from_absent() {
-        let tiny = solid(Color::rgb(0xFF, 0x00, 0x00), 1, 2);
-        let (_, provenance) = resolve_source(
-            Some(ImageInput::Undecodable),
-            Some(ImageInput::pixels(&tiny, 1, 2)),
-            None,
-            SchemeFlavor::Dark,
-        );
-        assert_eq!(provenance.level, Level::Preset);
-        assert_eq!(provenance.background(), Outcome::Failed(Skip::Undecodable));
-        assert_eq!(
-            provenance.wallpaper(),
-            Outcome::Failed(Skip::TooSmall {
-                width: 1,
-                height: 2
-            })
-        );
-        assert_eq!(
-            provenance.hint_key(),
-            Some("palette.hint.background.undecodable")
-        );
-        // 从没选过卡 → 出厂默认。
-        assert_eq!(provenance.card, CardKey::Sky);
-    }
-
-    #[test]
-    fn nothing_provided_uses_default_card_without_hint() {
-        let (scheme, provenance) = resolve_source(None, None, None, SchemeFlavor::Light);
-        assert_eq!(provenance.level, Level::Preset);
-        assert_eq!(provenance.preset(), Outcome::Used);
-        assert!(!provenance.did_fall_back());
-        assert!(provenance.adjustments.is_empty());
-        assert!(provenance.hint_key().is_none());
-        assert_eq!(scheme, DEFAULT_CARD.scheme(SchemeFlavor::Light));
-    }
-
-    #[test]
-    fn preset_selection_decides_the_fallback_colour() {
-        // 反证 2：回落落点是「用户选的那张卡」，不是默认卡。
-        let gray = solid(Color::rgb(0x80, 0x80, 0x80), 4, 4);
-        let (scheme, provenance) = resolve_source(
-            Some(ImageInput::pixels(&gray, 4, 4)),
-            None,
-            Some(CardKey::Rose),
-            SchemeFlavor::Light,
-        );
-        assert_eq!(provenance.level, Level::Preset);
-        assert_eq!(provenance.card, CardKey::Rose);
-        assert_eq!(scheme, CardKey::Rose.card().scheme(SchemeFlavor::Light));
-    }
-
-    #[test]
-    fn background_beats_wallpaper_when_both_work() {
-        let bg = solid(Color::rgb(0xB2, 0x6A, 0x00), 8, 8);
-        let wp = solid(Color::rgb(0x00, 0x79, 0x6B), 8, 8);
-        let (_, provenance) = resolve_source(
-            Some(ImageInput::pixels(&bg, 8, 8)),
-            Some(ImageInput::pixels(&wp, 8, 8)),
-            Some(CardKey::Teal),
-            SchemeFlavor::Dark,
-        );
-        assert_eq!(provenance.level, Level::BackgroundImage);
-        assert_eq!(provenance.wallpaper(), Outcome::Absent, "② 根本没轮到");
-        assert!(!provenance.did_fall_back());
-    }
-
-    #[test]
-    fn transparent_background_reports_its_own_reason() {
-        let transparent = solid(Color::rgba(0x3C, 0x8D, 0xBC, 0), 4, 4);
-        let (_, provenance) = resolve_source(
-            Some(ImageInput::pixels(&transparent, 4, 4)),
-            None,
-            None,
-            SchemeFlavor::Light,
-        );
-        assert_eq!(
-            provenance.background(),
-            Outcome::Failed(Skip::NoOpaquePixels)
-        );
-        assert_eq!(
-            provenance.hint_key(),
-            Some("palette.hint.background.no-opaque-pixels")
-        );
-    }
-
-    #[test]
-    fn a_five_percent_accent_drives_level_one() {
-        // 端到端反证（与 `palette::seed::tests::end_to_end_picks_the_five_percent_accent` 配对）：
-        // 95% 米白 + 5% 珊瑚的背景图**必须**在一级取色成功，且种子里就是那 5% 的珊瑚。
-        // 若 `analyze_rgba` 退化成「不透明像素求算术平均」，这条会变成「回落色卡」而红。
-        let beige = Color::rgb(0xF5, 0xF1, 0xEA);
-        let coral = Color::rgb(0xFF, 0x6B, 0x5A);
-        let mut pixels = Vec::with_capacity(40 * 40 * 4);
-        for index in 0..40 * 40 {
-            let color = if index < 40 * 40 - 80 { beige } else { coral };
-            pixels.extend_from_slice(&[color.r, color.g, color.b, color.a]);
-        }
-
-        let (scheme, provenance) = resolve_source(
-            Some(ImageInput::pixels(&pixels, 40, 40)),
-            None,
-            Some(CardKey::Rose),
-            SchemeFlavor::Light,
-        );
-        assert_eq!(provenance.level, Level::BackgroundImage, "{provenance:?}");
-        assert_eq!(provenance.background(), Outcome::Used);
-        let seed = provenance.seed.expect("一级成功必须带种子");
-        assert_eq!(seed.color, coral, "种子应当是 5% 的珊瑚");
-        assert!(provenance.hint_key().is_none());
-        // 方案来自珊瑚，而不是「回落到用户选的 Rose 卡」——两者 primary 不同。
-        assert_ne!(scheme, CardKey::Rose.card().scheme(SchemeFlavor::Light));
-        assert!(crate::palette::wcag::check(&scheme).is_clean());
-    }
-}
+mod tests;