Add CP437/ANSI-BBS support, SGR attributes, and BBS compatibility fixes

CP437/ANSI-BBS terminal emulation:
- Full 256-entry CP437 lookup table mapping each byte to ASCII, Mac Roman,
  or QuickDraw glyph rendering
- cp437_mode in terminal parser bypasses UTF-8 decoder for raw byte
  rendering, essential for BBS art using 0x80-0x9F range (C1 collision)
- ANSI-BBS terminal type in TTYPE negotiation, settings, preferences, and
  connect dialog — follows BBS community convention (ansi-bbs = CP437)
- UTF-8 mode CP437 fallback for BBSes that send raw CP437 despite
  xterm-256color TTYPE: flush incomplete sequences, route stray
  continuation bytes, detect unlikely 2-byte decodes, handle 0xF8-0xFF
- Bold-to-bright color promotion (SGR 1;30m = bright black, not invisible)

Glyph rendering expansion:
- 57 new QuickDraw primitives for double-line box drawing, mixed
  junctions, smileys, Greek/math symbols
- 18 fractional block elements (U+2581-U+258F, U+2594-U+2595, quadrants)
- 60 sextant characters (U+1FB00-U+1FB3B) as 2x3 grid patterns

SGR attribute rendering:
- True italic (TextFace), strikethrough (horizontal line at mid-height)
- Dim (SGR 2) halves foreground RGB on color systems
- Blink (SGR 5/6) promotes background to bright colors (DOS-style)

DNS and networking:
- Fix DNS resolution through NAT by removing source IP validation
  (Snow's DaynaPORT rewrites UDP source IPs)
- Add DNS-over-TCP fallback for NAT environments
- Fix backspace to send BS (0x08) instead of DEL (0x7F) for BBS compat

Performance and cleanup:
- Eliminate double cell fetch in draw_row (~80K cycles/redraw saved)
- Replace dark shade MULU with shift to avoid 68000 multiply
- Remove 50 lines of dead code superseded by glyph tables

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ecliptik

CMakeLists.txt

@@ -17,6 +17,7 @@ add_application(Flynn
     src/dns.c
     src/glyphs.c
     src/color.c
+    src/cp437.c
     src/macutil.c
     src/sysutil.c
     src/settings.c