Clarify std_register_cleanup_path batch registration semantics

[codeforester]

Context

std_register_cleanup_path validates all provided paths before registering any of them. If one path is unsafe, the function returns 1 immediately and none of the valid paths from that call are registered.

This all-or-nothing behavior may be intentional, but it is not explicit enough in the public contract and can surprise callers that pass mixed cleanup paths.

Failure Scenario

std_register_cleanup_path "$good_tmpdir" "/" "$also_good_tmpdir"

The function rejects / and returns 1. Neither valid temp path is registered for cleanup. The error names the unsafe path but does not make the batch semantics clear.

Scope

• Decide whether cleanup path batch registration should be all-or-nothing or best-effort per path.
• Implement the chosen behavior consistently.
• Document the behavior in lib/bash/std/README.md and function comments.
• Add BATS coverage for mixed valid and unsafe cleanup path arguments.

Acceptance Criteria

• Mixed valid/unsafe cleanup path calls have documented behavior.
• Tests prove whether valid paths are registered or skipped when the batch contains an unsafe path.
• Existing dangerous-path protections remain intact.
• ./tests/validate.sh passes.

Validation

• ./tests/validate.sh