fix(interpreter): shift builtin now updates positional parameters

[chaliy]

Summary

• shift builtin set _SHIFT_COUNT marker variable but interpreter never consumed it
• Added post-processing after builtin execution to drain positional parameters from call frame
• Fixes infinite loop in while [[ $# -gt 0 ]]; do case ... shift 2 ... esac; done pattern

Test plan

• issue_290_while_case_shift_loop — while/case with shift 2 terminates correctly
• issue_290_shift_1_default — default shift (shift 1) works

Closes #290