Harden std_run timeout retry behavior

[codeforester]

Summary

• cache timeout/gtimeout discovery once per std_run call instead of once per retry attempt
• make fallback timeout setup failures return 1 instead of command-not-found 127
• kill the fallback timer with SIGKILL after the command exits so its EXIT trap cannot remove the timeout marker first
• log Adding section to ... when update_file_section appends a new managed section

Fixes #112.

Validation

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