feat(style): name the four type roles

Body, Sender, Header, Meta. Three of them share the base size, which looks like
an oversight and is not: the sender is set apart by weight and the header by
small caps with wide tracking. Neither of those is a size, and solving them with
size instead would turn the log into a ransom note. Only meta steps down, because
it is meant to be skipped over rather than read.

Factors sit in a static array rather than as consts. The master spec puts
typography under theme control, and ThemeTypography is already the declared
extension point for it -- a const would wall that off before anyone gets there.

No caller yet. FontManager takes the first one in the next commit; that is the
one place in this cycle where a piece lands before its call site, and it closes
inside the same block.

HellionChat/Ui/StyleEngine/TypeScale.cs

@@ -0,0 +1,42 @@
+namespace HellionChat.Ui.StyleEngine;
+
+internal enum TypeRole
+{
+    Body,
+    Sender,
+    Header,
+    Meta,
+}
+
+// Named sizes derived from one base, so a role means the same thing wherever it
+// is drawn.
+//
+// Three of the four share the base size. That is deliberate: what sets the
+// sender apart is weight and what sets the header apart is small caps with wide
+// tracking, and neither is a size. Solving those with size instead would make
+// the log look like a ransom note. Only the meta role -- timestamps and the
+// header's trailing detail -- steps down, because it is meant to be skipped over
+// rather than read.
+//
+// The factors are defaults, not constants. The master spec puts typography under
+// theme control rather than user control, and ThemeTypography already exists as
+// the extension point for exactly that. A const would wall it off. What is
+// deliberately absent either way is a user-facing slider per role.
+internal static class TypeScale
+{
+    // Below this a timestamp stops being readable at any display scale.
+    internal const float MinPt = 7f;
+
+    private static readonly float[] Factors =
+    [
+        1.00f, // Body   -- the reference every other role is stated against
+        1.00f, // Sender -- set apart by weight, see FontManager.SenderWeight
+        1.00f, // Header -- set apart by small caps and tracking
+        0.82f, // Meta   -- timestamps, and the world and clock in the header
+    ];
+
+    internal static float FactorOf(TypeRole role) => Factors[(int)role];
+
+    internal static float SizePtOf(TypeRole role, float basePt) =>
+        TypeScaleMath.Resolve(basePt, FactorOf(role), MinPt);
+}