Summary by Sourcery

Improve the fnm installation script’s portability and robustness while updating a Rust dependency version.

Bug Fixes:

• Fix install script incompatibilities with POSIX /bin/sh by avoiding bash-specific syntax and utilities.
• Ensure temporary download directories are always cleaned up and handle failures to create them gracefully.

Enhancements:

• Add a portable temporary directory creation helper and use more robust command detection in the install script.
• Simplify shell configuration sourcing guidance and modernize fnm env evaluation in generated shell config.

Chores:

• Bump the indoc Rust crate dependency to version 2.0.7.