Exit/return codes not truncated to 8-bit range

[chaliy]

Description

Bash truncates exit and return codes to an unsigned 8-bit range (0-255). Bashkit does not perform this truncation, passing through values > 255 or negative values directly.

Scope

• exit 256 should produce exit code 0 (256 mod 256)
• exit 257 should produce exit code 1
• exit -1 should produce exit code 255 (two's complement wrap)
• exit -2 should produce exit code 254
• Same behavior for return in functions
• '' as a command should fail (exit code 127), not succeed

Repro

f() { return 256; }; f; echo $?
# bashkit outputs: 256 (wrong)
# expected: 0

f() { return -1; }; f; echo $?
# bashkit outputs: -1 (wrong)
# expected: 255

if ''; then echo TRUE; else echo FALSE; fi
# bashkit outputs: TRUE (wrong)
# expected: FALSE

Test coverage

9 skipped tests in crates/bashkit/tests/spec_cases/bash/exit-status.test.sh (PR #351).

Oils reference: https://github.com/oilshell/oil/blob/master/spec/exit-status.test.sh