PHLEGM v8 | Sequential Developer Checkpoint 28 | SuperPET 6809 ASCII Page

This builds directly on checkpoint 27 and retains the entire v8 developer distribution.

1. Adds a generic, data-driven display character-bank selection mode: fixed.
   Syntax: display_N_character_bank_select_mode=fixed plus
           display_N_character_bank_fixed_offset=$0800.
   Existing masked-match selector functionality is unchanged for PET machines.
2. SuperPET-9000 primary 6809 display selects the Waterloo ASCII 1KiB
   character set ($0800 in the 4096-byte CHARACTER ROM), not the PET sets.
   SuperPET 901640-01 ROM pages: PET $0000/$0400, ASCII $0800, APL $0C00.
   This addresses the screenshot's screen RAM predominantly containing ASCII
   spaces ($20) while rendering repeating PET graphics glyphs.
3. Adds generic MACHINE SELFTEST directive expect_character_bank_offset_N,
   with the SuperPET startup check requiring display 0 bank $0800.
4. MACHINE TRACE reports selector=fixed and the selected bank/glyph data.
5. Preserves the cfg-driven CPU/machine model and platform/back-end matrix.

No ROM firmware bundled or changed. This environment has no native FreeBASIC
compiler, and a live user-firmware boot was not tested. The MC6809 Waterloo
boot path requires further hands-on verification after compilation.

Documentation reference: VICE docs and Zimmers.NET SuperPET firmware listings.
