feat(theme): tabular figures on the roles that carry a running number

M0 said the big-readout typography would be settled once there was a
stopwatch to settle it against. `ClockulaTypography` now puts `tnum` on the
display, headline and title roles, which is every role a counting number
uses, and `ClockulaReadoutDefaults` names the three sizes the readouts share.

Digits stop changing width as they change, so the timer row, the setup panel
and the live pill stop twitching too — none of them needed a call-site
change to get it.

app/src/main/java/de/jeanlucmakiola/clockula/ui/theme/Type.kt

@@ -1,10 +1,71 @@
 package de.jeanlucmakiola.clockula.ui.theme
 
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Typography
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.ReadOnlyComposable
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.font.FontWeight
+
+/** Fixed-width digits. One string, one place, and a build rule that keeps it there (M7 D12). */
+private const val TABULAR_FIGURES = "tnum"
+
+private fun TextStyle.tabular(): TextStyle = copy(fontFeatureSettings = TABULAR_FIGURES)
 
 /**
- * Default Material 3 Expressive typography. A clock app is mostly one enormous
- * number, so the readout scale is the real typographic decision here — it gets
- * settled against a working stopwatch in M7, not guessed at in the scaffolding.
+ * The Material 3 Expressive default with **tabular figures** on the display,
+ * headline and title roles. A clock app is mostly one enormous number, and a
+ * readout whose glyph columns change width as its digits change jitters on
+ * every tick — so every existing readout (the timer row's `headlineLarge`, the
+ * setup panel's `displayMedium`, the live pill's `titleMedium`) is fixed here,
+ * with no call-site change anywhere.
+ *
+ * Body and label roles keep proportional figures: prose is not a column of
+ * numbers, and proportional digits read better inside a sentence.
  */
-val ClockulaTypography = Typography()
+val ClockulaTypography: Typography = Typography().run {
+    copy(
+        displayLarge = displayLarge.tabular(),
+        displayMedium = displayMedium.tabular(),
+        displaySmall = displaySmall.tabular(),
+        headlineLarge = headlineLarge.tabular(),
+        headlineMedium = headlineMedium.tabular(),
+        headlineSmall = headlineSmall.tabular(),
+        titleLarge = titleLarge.tabular(),
+        titleMedium = titleMedium.tabular(),
+        titleSmall = titleSmall.tabular(),
+    )
+}
+
+/**
+ * The three roles a readout uses, named once for the whole app — M0's written
+ * promise that the big-readout scale would be settled against a working
+ * stopwatch rather than guessed at in the scaffolding (M7 D12).
+ *
+ * The M3 `*Defaults` shape — composable getters over `MaterialTheme.typography`
+ * rather than constants — so a theme change still flows through them. No new
+ * font, no size outside the M3 scale, and no weight above Medium: `PLAN.md` §8's
+ * "Expressive ≠ big or bold" applies hardest to the one screen most tempted to
+ * break it.
+ */
+object ClockulaReadoutDefaults {
+
+    /** The stopwatch's big number. */
+    val Hero: TextStyle
+        @Composable @ReadOnlyComposable
+        get() = MaterialTheme.typography.displayLarge.copy(fontWeight = FontWeight.Medium)
+
+    /**
+     * The hundredths beside [Hero], about half its size. A readout whose
+     * hundredths are as large as its seconds makes the fastest-changing glyphs
+     * the loudest ones and drags the eye off the number that matters (D5).
+     */
+    val HeroFraction: TextStyle
+        @Composable @ReadOnlyComposable
+        get() = MaterialTheme.typography.headlineMedium
+
+    /** A figure inside a table row — the one place body-scale text *is* a column of numbers. */
+    val Figure: TextStyle
+        @Composable @ReadOnlyComposable
+        get() = MaterialTheme.typography.bodyLarge.tabular()
+}