Make cleanup dispatcher resilient to control flow in pre-existing EXIT traps

[codeforester]

Problem

At main commit a107134, __std_run_cleanup_hooks__ evaluates the pre-existing EXIT trap before registered hooks and paths. Because the saved command is evaluated inside the dispatcher function, a trap containing return returns from the dispatcher and skips all registered cleanup. A trap containing exit terminates the shell before library cleanup runs.

Both behaviors were reproduced: a registered hook did not create its marker after either trap 'return 0' EXIT or trap 'exit 7' EXIT was installed before registration.

The current composable-cleanup contract should not allow control flow in an older trap to bypass later registered cleanup.

Scope

• Isolate or sequence the original trap so return and exit cannot skip registered hooks and paths.
• Preserve the original trap's observable side effects and intended final exit status.
• Define and document execution order and status precedence.
• Retain support for multiline commands and embedded single quotes.

Acceptance Criteria

• Registered hooks and paths run when the original trap contains return.
• Registered hooks and paths run before the shell honors an explicit exit status from the original trap.
• Normal, multiline, and quoted original traps continue to run exactly once.
• The incoming shell exit status remains available to the original trap and cleanup hooks.

Validation

• bats lib/bash/std/tests/lib_std.bats
• ./tests/validate.sh
• git diff --check

[codeforester]

Implemented by merged PR #172. The cleanup dispatcher now isolates pre-existing EXIT-trap control flow so return and exit cannot skip registered hooks or paths, with coverage for both cases in lib/bash/std/tests/lib_std.bats.