Export a sourceable library version constant

[codeforester]

Context

The repository tracks its release version in the root VERSION file, but scripts that source the library cannot programmatically verify which version they loaded.

A readonly version constant would help downstream scripts enforce minimum behavior when adopting the library.

Scope

• Add a sourceable public version constant, such as BASE_BASH_LIBS_VERSION or per-library constants if that better matches the design.
• Keep the value aligned with the repo root VERSION file.
• Document the constant in the README and stdlib README.
• Add tests that confirm the exported value exists and matches the expected version source.

Acceptance Criteria

• Consumers can check the loaded library/package version after sourcing lib_std.sh.
• The version source of truth is clear and tested.
• Release/version docs mention the exported constant.

Validation

• ./tests/validate.sh

Suggested Branch

enhancement/<issue>-20260618-library-version-export