Expose stdlib loaded guard for companion libraries

[codeforester]

Context

Companion libraries currently detect lib_std.sh indirectly by checking for selected functions. That works, but it is an implicit contract. As more companion libraries are added, they should have a stable, documented stdlib-loaded marker to check before sourcing.

Scope

Add a Base-native loaded guard marker from lib_std.sh, then use it from companion libraries. Do not borrow code from external shell libraries.

Acceptance Criteria

• lib_std.sh exposes a documented loaded marker after successful initialization.
• lib_file.sh, lib_git.sh, and any current companion libraries check the stable marker before loading.
• Sourcing a companion library before lib_std.sh fails clearly with the existing style of error message.
• Existing source-once behavior remains unchanged.
• README or library docs describe the source order/guard contract.
• BATS coverage verifies guarded and correctly-loaded paths.

Validation

• Focused BATS coverage for stdlib and companion-library guard behavior.
• ./tests/validate.sh
• `git diff --check