Fix semantic foreground mapping to ANSI color7 across mappers

[OldJobobo]

Summary

This changes Aether’s semantic role mapping so foreground follows ANSI
color7 (white/default foreground), not color15 (bright white).

It also aligns related defaults to color7:

• cursor
• selection_background

ANSI slot mappings remain unchanged (color15 is still bright white).

Changes

• Updated role mapping in CLI mapper:
  • src/cli/utils/color-mapper.js
• Updated role mapping in app state:
  • src/state/ThemeState.js
• Updated role mapping in synthesizer:
  • src/components/ColorSynthesizer.js
• Aligned default semantic values:
  • src/constants/colors.js
• Updated docs for extended/Base16 and ANSI terminology:
  • docs/base16.md
  • docs/color-extraction.md

Why

Aether was assigning semantic foreground from slot 15, which conflicts
with ANSI/Base16 semantics where default foreground text corresponds to
slot 7 and slot 15 is bright white.

Verification

• Confirmed all role-mapper paths now map:
  • foreground -> color7
  • cursor -> color7
  • selection_background -> color7
• Confirmed no mapper path in src/ still assigns these from
  palette[15].

Notes

• selection_foreground remains unchanged.
• npm run format:check could not run in this environment because
  prettier is not installed (prettier: command not found).

[bjarneo]

LGTM.

Thank you!