Implement bold-color support (bright and fixed)

[mgeisler]

Add support for Ghostty's bold-color configuration, allowing bold text to be rendered using bright colors or a specific fixed color.

Summary of changes:

• src/terminal.zig: Add BoldConfig and bold_config field to Terminal.
• src/module.zig: Add ghostel--set-bold-config to update bold settings.
• src/render.zig: Implement color substitution logic in readCellProps. Bold text with palette colors 0-7 is mapped to 8-15. Bold text with default foreground uses bold_config.fixed_color if mode is fixed.
• lisp/ghostel.el: Add ghostel-bold-color defcustom (default bright) and logic to apply it to new and existing terminals.
• test/ghostel-test.el: Add unit tests for both 'bright and fixed-color bold modes.
• src/emacs.zig: Add bright symbol to cached symbols.

Matches Ghostty 1.2.0 behavior.

Written by Gemini, tested by me.

[emil-e]

I'll have another look tomorrow!

[emil-e]

LGTM

@dakra Any thoughts? I personally don't quite understand the point but given Ghostty proper has this, I'm guessing it's a thing that people want? :)

[dakra]

LGTM

lgtm. Thanks, merging.

@dakra Any thoughts? I personally don't quite understand the point but given Ghostty proper has this, I'm guessing it's a thing that people want? :)

Personally I'm also happy with just a bold face but I also don't have any aesthetic taste ;)
I asked the LLM why someone would want that option and the reasons make sense to me:

• Legacy/theme convention. Old terminals couldn't render bold glyphs, so "bold" historically meant "switch to
  the bright palette slot." Most color schemes (Solarized, Gruvbox, Nord, etc.) were tuned assuming bold red =
  bright red — without the brightening, output from ls, grep, prompts, etc. looks duller than the theme author
  intended.
• Visual punch. Bold-weight alone can be subtle at small sizes or with thin fonts; the color shift makes
  emphasized tokens actually pop.
• Faux-bold avoidance. Some users disable bold-weight (eyestrain, ligature issues, fonts without a real bold
  face) but still want emphasis — bold-color = "#hex" gives them a single accent color instead of synthetic stroke
  widening.