Enforce indexed-array contracts in list and string helpers

[codeforester]

Context

lib_list.sh and the array-oriented parts of lib_str.sh document caller-owned indexed arrays, but they currently validate only the variable name. Passing a scalar can silently coerce it into an indexed array, and passing an associative array reads values in hash order.

lib_arg.sh already has a local pattern for checking declared array kind before mutating caller-owned variables.

Scope

• Add or reuse a standard helper for checking declared Bash array kind.
• Require indexed arrays for list_append, list_prepend, list_remove, list_contains, list_unique, list_length, str_split, str_join, and str_in_array where the argument is documented as an array.
• Preserve valid indexed-array behavior and in-place mutation semantics.
• Add BATS coverage for scalar and associative-array rejection.
• Update docs if callers must explicitly declare -a result arrays before use.

Acceptance Criteria

• List and string array helpers reject scalar variables and associative arrays with clear diagnostics.
• Existing valid indexed-array examples continue to pass.
• The implementation follows existing Bash 4.2-compatible style and avoids nameref requirements.
• ./tests/validate.sh passes.