tests: cover leading empty fields in base_str_split

[codeforester]

Goal

Add focused regression coverage for a leading empty field produced by base_str_split.

Background

The existing BATS tests cover empty fields between repeated separators and after a trailing separator, but do not cover a value that starts with the separator. The helper currently preserves this field; documenting and testing that boundary makes the behavior clear to new users.

Scope

Add one BATS case in lib/bash/str/tests/lib_str.bats and a concise behavior note in lib/bash/str/README.md. Do not change production behavior or public signatures.

Acceptance Criteria

• Splitting ,alpha,beta on , produces three elements: empty string, alpha, and beta.
• The string-library README states that leading empty fields are preserved.
• The focused string-library BATS file and full repository validation pass.

Validation

Run bats lib/bash/str/tests/lib_str.bats and ./tests/validate.sh.

Non-Goals

No API or implementation change.

Project Fields

• Status: Ready
• Priority: P3
• Area: CI
• Initiative: Adoption Polish
• Size: S
• Milestone: v2.2.0
• Target date: unset.

Agent Assignment

Unassigned and open to first-time contributors.