fix(builtins): cap AWK output buffer size to prevent memory exhaustion

[chaliy]

Summary

• Add MAX_AWK_OUTPUT_BYTES constant (10MB) to cap total AWK output
• write_output() now checks total across stdout, stderr, file_outputs, and file_appends
• Returns error with exit code 2 when limit exceeded
• Prevents OOM from awk '{for(i=0;i<100000;i++) printf "%10000s\n", "x"}'

Test plan

• Stdout overflow caught with exit code 2
• Normal output under limit works correctly
• File redirect overflow also caught
• All existing AWK tests pass

Closes #987