Harden stdlib range validation

[codeforester]

Summary

• Refactor assert_integer_range to validate literal bounds directly instead of relying on Bash dynamic scoping.
• Add focused BATS coverage for invalid range bounds and clearer bound-specific error messages.
• Remove a production # FIX: development comment and guard against future # FIX: comments in validation.
• Document the stdlib root-layout assumption and make the Bash 4.2+ requirement prominent in the README.

Validation

• bats lib/bash/std/tests/lib_std.bats --filter "assert_integer_range rejects non-integer bounds directly"
• bats lib/bash/std/tests/lib_std.bats lib/bash/file/tests/lib_file.bats
• ./tests/validate.sh
• git diff --check

Fixes #41