fix(SchemaBuilder): allow timestamps in alter statements

[elpete]

Summary

• queue both createdDate and modifiedDate columns when timestamps() is used in an alter callback
• preserve the existing create-table behavior
• cover generated alter SQL for all six supported schema grammars

Root cause

timestamps() appended two columns and returned the blueprint. Unlike single-column helpers, it could not be passed to addColumn(), and an alter blueprint did not automatically queue either appended column. As a result, there was no supported way to add both timestamp columns with the shortcut.

Validation

• reproduced first with a failing regression test: expected 2 alter statements, received 0
• schema suite: 836 passed
• full suite on Lucee 6.2.7.16: 2,586 passed, 0 failed, 0 errors, 3 skipped
• box run-script format:check
• git diff --check

Fixes #310