feat: palette-coherent showcase presets (accent-to-palette-stop, rework Aurora)

Keep every temporal-colored cell on the active palette's own gradient, per
the requirement that added front colors must agree with the palette:

- Accent-mode default accent now samples the palette's own vivid stop
  (palette_accent_color, brightness 0.85) instead of the hot end (1.0), which
  is white/desaturated on many palettes (Slime, Cosmic) and washed the front.
- All four showcase presets use Accent mode with palette-derived accents
  (temporal_accent: None); dropped the foreign hand-picked gold/cyan and the
  off-palette hue-mode drift (Aurora/Bloom were hue).
- Rework Aurora: deep Ocean palette + restrained afterglow (0.15) + higher
  white-point (max_brightness 26) so the luminous network reads against a dark
  body instead of washing to a pale fill.

Regenerates the 4 showcase goldens and temporal_color_accent (now reflects the
palette-derived accent). All existing presets remain byte-identical.

Author: tamirelazar

src/render/palette.rs

@@ -2786,10 +2786,13 @@ fn temporal_modulate(
             oklch_to_srgb(oklch.l, oklch.c, h)
         }
         TemporalMode::Accent => {
-            // Front accent = the palette's hot end (brightness 1.0). Blend toward it
-            // for the growing front (blend > 0). Mix in OKLch for perceptual evenness.
+            // Front accent defaults to the palette's own vivid bright stop
+            // (brightness 0.85), NOT the hot end (brightness 1.0) — many palettes
+            // desaturate to white/gray at 1.0, which gives a washed, off-palette
+            // front. Sampling the palette keeps the accent on its own gradient.
+            // A hand-picked `accent` (CLI/preset) overrides this.
             let accent =
-                accent.unwrap_or_else(|| map_brightness_rgb(1.0, palette, false, false, 0.0, None));
+                accent.unwrap_or_else(|| palette_accent_color(&palette, false, false, 0.0, None));
             let t = blend.max(0.0) * strength;
             mix_oklch(base, accent, t)
         }
@@ -2939,16 +2942,21 @@ mod tests {
             Palette::Slime,
             Some(custom),
         );
-        let hot_end = temporal_modulate(base, 0.9, TemporalMode::Accent, 1.0, Palette::Slime, None);
+        let palette_default =
+            temporal_modulate(base, 0.9, TemporalMode::Accent, 1.0, Palette::Slime, None);
         assert_ne!(
-            with_custom, hot_end,
-            "custom accent must differ from hot-end"
+            with_custom, palette_default,
+            "custom accent must differ from the palette-derived accent"
         );
-        // None path must equal the pre-change behavior (hot-end accent).
-        let manual_accent = map_brightness_rgb(1.0, Palette::Slime, false, false, 0.0, None);
+        // The None path blends toward the palette's own vivid stop
+        // (`palette_accent_color`, brightness 0.85), NOT the hot end (1.0).
+        let palette_accent = palette_accent_color(&Palette::Slime, false, false, 0.0, None);
         let t = (TEMPORAL_TANH_K * 0.9_f32).tanh() * 1.0;
-        let expect_none = mix_oklch(base, manual_accent, t);
-        assert_eq!(hot_end, expect_none, "None must reproduce hot-end accent");
+        let expect_none = mix_oklch(base, palette_accent, t);
+        assert_eq!(
+            palette_default, expect_none,
+            "None must blend toward palette_accent_color"
+        );
     }
 
     #[test]

src/render_art_defaults.rs

@@ -59,23 +59,27 @@ impl From<Preset> for RenderArtDefaults {
     fn from(preset: Preset) -> Self {
         use crate::render::charset::{Charset, GlyphConfig, GlyphSelection};
         use crate::render::palette::{
-            IntensityMapping, Palette, PaletteCycle, PaletteCycleMode, RgbColor, TemporalMode,
+            IntensityMapping, Palette, PaletteCycle, PaletteCycleMode, TemporalMode,
         };
+        // All showcase presets use Accent mode with `temporal_accent: None`, so
+        // the growing front blends toward the palette's OWN vivid stop
+        // (`palette_accent_color`, brightness 0.85) — keeping every rendered
+        // color on the palette gradient (no foreign hand-picked hues, no
+        // off-palette hue rotation).
         match preset {
             Preset::Lumen => Self {
                 temporal_color: 0.5,
                 temporal_lag_frames: 8.0,
                 temporal_mode: TemporalMode::Accent,
-                temporal_accent: Some(RgbColor::from_hex(0xffb347)), // warm gold
                 palette: Some(Palette::Slime),
                 intensity_mapping: IntensityMapping::smoothstep(),
                 ..Self::default()
             },
             Preset::Aurora => Self {
-                temporal_color: 0.3,
+                temporal_color: 0.4,
                 temporal_lag_frames: 12.0,
-                temporal_mode: TemporalMode::Hue,
-                palette: Some(Palette::Cosmic),
+                temporal_mode: TemporalMode::Accent,
+                palette: Some(Palette::Ocean),
                 intensity_mapping: IntensityMapping::smoothstep(),
                 ..Self::default()
             },
@@ -86,13 +90,12 @@ impl From<Preset> for RenderArtDefaults {
                     mode: PaletteCycleMode::Mirror,
                 },
                 temporal_color: 0.2,
-                temporal_mode: TemporalMode::Hue,
+                temporal_mode: TemporalMode::Accent,
                 ..Self::default()
             },
             Preset::Etching => Self {
                 temporal_color: 0.4,
                 temporal_mode: TemporalMode::Accent,
-                temporal_accent: Some(RgbColor::from_hex(0x00e5ff)), // cyan
                 palette: Some(Palette::Neon),
                 charset: Some(Charset::Braille),
                 glyph: GlyphConfig {
@@ -155,9 +158,31 @@ mod tests {
             lumen.temporal_mode,
             crate::render::palette::TemporalMode::Accent
         );
-        assert!(lumen.temporal_accent.is_some());
+        // Showcase presets blend toward their OWN palette's vivid stop, so the
+        // accent is intentionally None (palette-derived, not a foreign hue).
+        assert_eq!(lumen.temporal_accent, None);
         assert_eq!(lumen.palette, Some(crate::render::palette::Palette::Slime));
 
+        // All showcase presets use palette-coherent Accent mode (no hue-mode
+        // drift off the gradient).
+        for p in [
+            Preset::Lumen,
+            Preset::Aurora,
+            Preset::Bloom,
+            Preset::Etching,
+        ] {
+            let d = RenderArtDefaults::from(p);
+            assert_eq!(
+                d.temporal_mode,
+                crate::render::palette::TemporalMode::Accent,
+                "{p:?} must use Accent mode"
+            );
+            assert_eq!(
+                d.temporal_accent, None,
+                "{p:?} accent must be palette-derived"
+            );
+        }
+
         let etching = RenderArtDefaults::from(Preset::Etching);
         assert_eq!(
             etching.charset,

src/simulation/config.rs

@@ -865,18 +865,21 @@ impl Preset {
                 }];
             }
             Preset::Aurora => {
-                // Exploratory base, slow decay, soft glow.
+                // Exploratory base, soft Gaussian glow on a deep Ocean palette.
+                // Moderate decay + restrained afterglow keep a dark body so the
+                // luminous network reads against it (avoids the washed-out fill
+                // that heavy afterglow + a low white-point produced).
                 config.sensor_angle = 45.0;
                 config.sensor_distance = 15.0;
                 config.rotation_angle = 60.0;
-                config.decay_factor = 0.96;
+                config.decay_factor = 0.92;
                 config.deposit_amount = 3.0;
                 config.diffusion_kernel = DiffusionKernel::Gaussian;
                 config.diffusion_sigma = 2.5;
                 config.diffuse_weight = 0.6;
-                config.afterglow = 0.4;
-                config.decay_gamma = 0.6;
-                config.max_brightness = 12.0;
+                config.afterglow = 0.15;
+                config.decay_gamma = 0.7;
+                config.max_brightness = 26.0;
                 config.species_configs = vec![SpeciesConfig {
                     name: "default".to_string(),
                     count: 30_000,