Plan graph: fit scales small graphs UP to use the panel (wave-2 item 1)

The fit zoom was capped at 1×, so any graph smaller than the panel
rendered at natural size, centered, with big empty margins above and
below — the common case for early-session chains in the 384px panel.
The fit now scales up to fill (FIT_ZOOM_MAX 1.5 — a lone node doesn't
become a billboard), floor-clamped as before. Math extracted into
fitZoomFor() and unit-tested (scale-up, cap, width/height-bound,
floor, unmeasured viewport).

next-env.d.ts

@@ -1,7 +1,7 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/types/routes.d.ts";
-import "./.next/types/root-params.d.ts";
+import "./.next/dev/types/routes.d.ts";
+import "./.next/dev/types/root-params.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.