refactor(bcs): improve BCS compliance and code safety

- Replace generic EOF here-doc delimiters with descriptive names
- Use (()) for integer comparisons per BCS0503
- Use ${var@Q} quoting in error messages per BCS0302
- Add -- separator to basename call for safety
- Add ||: to shift 2 to prevent set -e exit
- Change KISS expansion to "Keep It Simple, Silly"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Biksu-Okusi