Document verification of a tampered bundle

[lntutor]

Summary

• add a copy-safe walkthrough that modifies stdout.txt in a disposable bundle
• show the expected integrity failure, tampered-file status, and exit code
• clarify that the example does not bypass ReproSpec security boundaries

Validation

• npm test (command passes; repository currently discovers 0 tests)
• ran the documented capture, copy, tamper, and verify commands
• verified reprospec verify exits 1 and reports stdout.txt: TAMPERED
• git diff --check

Fixes #4

[lntutor]

Current head 227b3f94e4e8e97068ff36eb8f6b1178c0a35eec is mergeable. This PR is scoped to QUICKSTART.md, adding documentation for verifying a tampered bundle. There are currently no review comments or visible failing checks attached to this head. Please review when convenient.