fix(output): derive table columns from all row keys

[dumanoglu1]

Summary

• derive table headers from the union of keys across all array rows instead of only data[0]
• preserve existing output for homogeneous arrays
• add a regression test for fields that first appear on later rows

Fixes #46.

Testing

• npm run test -- test/output.test.ts
• npm run type-check
• npx biome check src/output.ts test/output.test.ts

Note: npm run test runs 30 suites successfully, including test/output.test.ts, but 5 existing auth/login suites fail in this local checkout with ERR_REQUIRE_ESM from @opensea/sdk requiring @noble/hashes/sha3.js. This is unrelated to the table formatter change.