The Dido class shows ships designed around a weapon

Is designing an entire vehicle around one weapon a postwar invention?

No; the Dido-class cruisers were designed around their 5.25-inch gun battery.

The Dido class provides a naval counterexample to the idea that weapon-centered vehicle design began after 1945. Its role and fighting arrangement were shaped by the decision to build around a 5.25-inch dual-purpose battery. The weapon requirement influenced the cruiser as a whole rather than being fitted after the main design choices were settled.