KeyJ's ZX Weave Pattern

Author:          KeyJ / TRBL
Category:        Christmas Challenge 2023
System:          Sinclair ZX Spectrum
Language:        Sinclair BASIC
Len source code: 106 bytes (BASIC as ASCII), 96 bytes (zmakebas source code)
Len exe file:    140 bytes (tape file)
Len code only:   115 bytes (raw BASIC dump)

Instructions:
- build with:
  $ zmakebas vc3_23_zx_zmakebas.bas -l -a 1 -s 1 -i 1 -n vc3_23kjZX -o vc3_23_zx.tap
  - using Russell Marks' zmakebas utility
    - can be installed on standard Ubuntu Linux systems with
      $ sudo apt install zmakebas
- run in basically any emulator by drag&drop of the .tap file or via
  command line
  - e.g., on Linux again:
    $ fuse vc3_23_zx.tap
  - if the emulator doesn't autostart the tape, enter the LOAD "" command
    manually
    - that's usually J, Ctrl+P, Ctrl+P, Enter

Description:
Uses a simple parametric formula: X and Y in, character out.

Comments:
Nothing special - same approach as with my Python implementation, except it
uses a different range for x (so the results never become negative), and
there's a workaround for Sinclair BASIC's lack of a built-in modulo operator.