build(nix): read version from VERSION file; add bump script
Centralize the package version in a top-level VERSION file so the
flake derivation and any future tooling (release automation, CLI
--version output) pull from one place. Adds scripts/bump-version.sh
for major/minor/patch bumps or an explicit version override.
Bumps version to 0.1.1 to cover the meta.mainProgram addition.