fix(tombstone-library): fix version.rb require and files path in gemspec template

[torreypayne]

Fixes the path to version.rb in gemspec-template.erb when tombstoning a gem.

Technical Context

In toys/tombstone-library/.toys.rb, generate_files writes the tombstone version constant to:

mkdir_p "lib/#{namespace_dir}"
generate_one "lib/#{namespace_dir}/version.rb", "version-template.erb"

However, gemspec-template.erb previously omitted the lib/ directory prefix:

• require File.expand_path("<%= namespace_dir %>/version", __dir__) -> require File.expand_path("lib/<%= namespace_dir %>/version", __dir__)
• gem.files = [..., "<%= namespace_dir %>/version.rb"] -> gem.files = [..., "lib/<%= namespace_dir %>/version.rb"]

Failure Modes Resolved

1. Gemspec Loading / Build Failures: Any evaluation of the generated gemspec (via rake build, bundle install, or Gem::Specification.load) previously attempted to require <gem_root>/<namespace>/version.rb, raising LoadError: cannot load such file.
2. Missing Version File in Package: Because gem.files did not reference lib/<namespace_dir>/version.rb, gem build failed to include version.rb in the packaged .gem archive.

Repro & Verification

# 1. Create dummy multi-namespace skeleton
mkdir -p my-sample-gem/lib/my/sample/gem
touch my-sample-gem/{README.md,LICENSE.md,.yardopts}
cat << 'EOF' > my-sample-gem/lib/my/sample/gem/version.rb
module My; module Sample; module Gem; VERSION = "1.0.0"; end; end; end
EOF

# 2. Testing unpatched template (fails with LoadError):
# require File.expand_path("my/sample/gem/version", __dir__)
ruby -e "Gem::Specification.load('my-sample-gem/my-sample-gem.gemspec')"
# => LoadError: cannot load such file -- .../my-sample-gem/my/sample/gem/version

# 3. Testing patched template (passes & builds package cleanly):
# require File.expand_path("lib/my/sample/gem/version", __dir__)
ruby -e "spec = Gem::Specification.load('my-sample-gem/my-sample-gem.gemspec'); puts spec.name + ' ' + spec.version.to_s"
# => my-sample-gem 1.0.0

[aandreassa]

You can also merge this as chore since we are not publishing anything! (it is not an actual gem)