Support beta tags for prebuilt releases

[benvinegar]

Summary

• add --tag <dist-tag> support to the prebuilt npm publish script
• thread a manual npm_tag input through the prebuilt release workflow
• keep tag-push releases defaulting to latest

Why

PR #14 landed the actual prebuilt npm release workflow, but after merge we still needed a safe beta path so we can publish something like 0.3.0-beta.1 under npm's beta dist-tag before moving stable users off latest.

What changed

• scripts/publish-prebuilt-npm.ts
  • parses --tag <name>
  • publishes with npm publish --tag <name>
  • reports the chosen dist-tag in dry-run / success output
• .github/workflows/release-prebuilt-npm.yml
  • adds workflow_dispatch.inputs.npm_tag
  • uses that tag for dry-run and publish steps on manual dispatch
  • still defaults tag pushes to latest

Validation

• bun run typecheck
• bun run build:prebuilt:artifact
• bun run stage:prebuilt:release
• bun run publish:prebuilt:npm -- --dry-run --tag beta