`versionCode` was hard-coded to 1, so every release looked like the
same version to Android. It is now derived from `VERSION` as
`major * 10000000 + minor * 10000 + patch` (0.1.62 gives 10062), which
is above the 1 existing installs have and keeps growing, so Android
never mistakes an update for a downgrade.
The formula only increases while patch stays below 10000 and minor
below 1000. `build.gradle` refuses a `VERSION` outside that range or
outside Android's 2100000000 ceiling, and `scripts/bump-version.mjs`,
which only ever bumps patch, refuses to create such a version before
anything is committed or tagged.