feat(identity): pure-black (AMOLED) dark theme

ColorScheme.toPureBlack() sets background, surface, surfaceDim and
surfaceContainerLowest to #000 and dims the other containers by scaling
their sRGB channels, so their hue tint carries over and grid tiles drawn
as containers over surface keep visible gaps. FloretExpressiveTheme takes
pureBlack to apply it to the dark scheme, dynamic or app-supplied.

core-prefs gains ThemeMode.BLACK (dark, pure black) and
ThemeMode.isPureBlack, stored under the same theme_mode key.

core-prefs/src/main/kotlin/de/jeanlucmakiola/floret/prefs/Appearance.kt

@@ -4,16 +4,22 @@ import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.flow.map
 
-/** Light/dark override. [SYSTEM] follows the device setting. */
-enum class ThemeMode { SYSTEM, LIGHT, DARK }
+/**
+ * Light/dark override. [SYSTEM] follows the device setting; [BLACK] is dark
+ * with pure-black surfaces (the AMOLED option).
+ */
+enum class ThemeMode { SYSTEM, LIGHT, DARK, BLACK }
 
 /** Resolve to a concrete dark flag; [SYSTEM] defers to [systemDark]. */
 fun ThemeMode.isDark(systemDark: Boolean): Boolean = when (this) {
     ThemeMode.SYSTEM -> systemDark
     ThemeMode.LIGHT -> false
-    ThemeMode.DARK -> true
+    ThemeMode.DARK, ThemeMode.BLACK -> true
 }
 
+/** Whether the theme should draw pure-black surfaces; only [ThemeMode.BLACK] does. */
+val ThemeMode.isPureBlack: Boolean get() = this == ThemeMode.BLACK
+
 /** The two appearance choices every app in the family has. */
 data class Appearance(
     val themeMode: ThemeMode = ThemeMode.SYSTEM,

core-prefs/src/test/kotlin/de/jeanlucmakiola/floret/prefs/ThemeModeTest.kt

@@ -42,6 +42,24 @@ class ThemeModeTest {
         assertThat(dark).isTrue()
     }
 
+    @Test
+    fun `isDark lets BLACK override a light system`() {
+        val mode = ThemeMode.BLACK
+
+        val dark = mode.isDark(systemDark = false)
+
+        assertThat(dark).isTrue()
+    }
+
+    @Test
+    fun `isPureBlack is set only for BLACK`() {
+        val modes = ThemeMode.entries
+
+        val pureBlack = modes.filter { it.isPureBlack }
+
+        assertThat(pureBlack).containsExactly(ThemeMode.BLACK)
+    }
+
     @Test
     fun `Appearance defaults to following the system with dynamic colour on`() {
         val appearance = Appearance()