Add list array helper library

[codeforester]

Summary

• Add lib/bash/list/lib_list.sh with in-place indexed-array mutation helpers and result-variable helpers.
• Cover append, prepend, remove, contains, unique, length, source guardrails, whitespace, empty values, and invalid variable names.
• Wire the new library into docs, standards, and repository validation.

Validation

• shellcheck --severity=error lib/bash/list/lib_list.sh lib/bash/list/tests/lib_list.bats
• bats lib/bash/list/tests/lib_list.bats
• ./tests/validate.sh
• git diff --check

Closes #70