Add benchmark spec for PackWizard shared URL

[alex-ross]

Summary

• Add an opt-in RSpec benchmark that measures PackwizardParser.parse_url against https://www.packwizard.com/s/sqRHbz2.
• Exclude benchmark specs from the default suite unless RUN_BENCHMARKS=1 is set.
• Document how to run the benchmark in the README.
• Update Bundler setup to use the gemspec and add a console entrypoint.

Testing

• Ran the full RSpec suite locally: 43 examples, 0 failures.
• Ran the new benchmark spec against the live PackWizard URL: completed successfully and reported the parse time.
• Ran standardrb on the changed Ruby files; the touched files passed.