Add stdlib dependency guards to companion libraries

[codeforester]

Context

lib_file.sh and lib_git.sh depend on stdlib functions such as log_error, log_debug, fatal_error, and log_info_file. Their READMEs document the load order, but direct standalone sourcing fails later with rough command not found errors.

Scope

• Add an early dependency guard to lib/bash/file/lib_file.sh.
• Add an early dependency guard to lib/bash/git/lib_git.sh.
• The guard should emit a concise message to stderr and return nonzero when the file is sourced without lib_std.sh.
• Keep double-sourcing behavior intact.
• Add BATS coverage for direct companion-library sourcing without stdlib.

Acceptance Criteria

• Sourcing each companion library without stdlib returns nonzero with a clear message.
• Sourcing after stdlib works as before.
• Double-sourcing remains a no-op success.

Validation

• ./tests/validate.sh

Suggested Branch

bug/<issue>-20260618-companion-dependency-guards