fix: explain malformed blueprint JSON errors

[11suixing11]

Summary

• report malformed blueprint JSON with the blueprint path, line/column when available, and the original parser error
• add a short validation hint instead of only printing Invalid Json in blueprint file
• add a npm test script with a node:test regression for the formatter

Fixes #30

Verification

• npm test
• npm run build
• npm run lint
• CLI smoke test with malformed JSON exits 1 and prints the path, line/column, parser error, and validation hint
• git diff --check

Note: npm ci reports 2 existing audit findings (1 low, 1 high); I did not change dependencies in this focused fix.

[devi5040]

Approved the PR.