Run test suite in CI; fix missing @types/node dependency

[cokehill]

What changed

1. CI never ran the test suite. .github/workflows/ci.yml
   already ran typecheck, lint, and build on every push/PR, but had
   no step running npm test — meaning the 21 unit tests added in a
   prior PR have never actually executed in CI. Added a "Run tests"
   step (npm test) between lint and build.

2. @types/node was missing from devDependencies. src/lib/stellar.ts
   uses Buffer and require, but the types this depends on were
   only present via transitive hoisting from another package — not
   pinned anywhere. That makes the DTS build non-reproducible: a
   different dependency resolution (e.g. in a clean CI runner) could
   silently fail. Added it explicitly.

Known follow-up (not fixed here, flagged for visibility)

The lint job in this same workflow fails on every run regardless
of this PR — there's no ESLint config file in the repo at all,
despite eslint being installed and a lint script defined.
Running npm run lint locally confirms this:
"ESLint couldn't find a configuration file."
Happy to follow up with a dedicated PR adding a basic ESLint config
if useful — didn't want to bundle an opinionated rule-set choice
into this PR.

Testing

• npx vitest run — 21/21 passing
• npm run build — CJS, ESM, DTS all succeed