Enforce fallback timeout kill bounds

[codeforester]

Summary

• add TERM-then-KILL escalation to the Bash timeout fallback
• suppress fallback wait-side kill job messages while preserving command stderr
• add regression coverage for commands that ignore TERM

Fixes #91

Validation

• bats --filter "std_run fallback timeout kills commands that ignore TERM" lib/bash/std/tests/lib_std.bats
• bats lib/bash/std/tests/lib_std.bats
• ./tests/validate.sh