Add unit-label markup: super/subscripts and stacked fractions

Lighter shorthand (^x, _x, {a/b}) rendered as glyph paths in the ГОСТ
font — no LaTeX engine, so typography and self-contained output are
preserved. Constructs nest (e.g. {m/s^2}, x^{1/2}). A marked-up unit is
sized to its true ink bounding box and centered in the unit box with
white space, fitting in both axes so tall fractions don't overflow.

Also fit plain units by measured glyph width rather than a per-character
estimate. Bump to 0.77.

k8s/deployment.yaml

@@ -15,7 +15,7 @@ spec:
     spec:
       containers:
         - name: m1730-generator
-          image: git.baumann.gr/adebaumann/m1730-generator:0.76
+          image: git.baumann.gr/adebaumann/m1730-generator:0.77
           imagePullPolicy: Always
           ports:
             - containerPort: 5000