Add tinted8 support

[JamyGolden]

I've been working on this for a while and I was slowly creating the spec as I was creating the builder. https://github.com/tinted-theming/home/blob/main/specs/tinted8/builder.md. This PR doesn't fully match the spec, but I'm going to update the spec since I think this PR introduces good changes to the spec.

Added

• Library: Initial Tinted8 support (tinted_builder::tinted8) with Scheme
  type and spec version constants (SUPPORTED_BUILDER_SPEC_VERSION,
  SUPPORTED_STYLING_SPEC_VERSION).
• Library: Tinted8 template rendering with nested scheme, palette, ui,
  and syntax contexts; color objects expose hex, hex-r/g/b, hex-bgr,
  rgb, rgb16, and dec fields.
• Library: Tinted8 palette expansion and derivation rules (normal/dim/bright
  variants; derive orange/brown; auto-generate gray when missing).
• CLI: Build support for Tinted8 schemes, including semver validation of
  supports.tinted8-styling and supports.tinted8-builder in
  templates/config.yaml.
• CLI: List generation for Base16/Base24 and Tinted8 via yaml template config
  options.list with schemes array provided to mustache.
• Docs: Add specs/builder.md and specs/styling.md references; expand
  READMEs with Tinted8 usage and template variables.
• Tests: Add Tinted8 unit tests in library and build-flow tests in CLI

Changed

• BREAKING: Library: Color API updated — Color::new now takes (hex, Option<ColorName>, Option<ColorVariant>); Color struct gained
  name and variant fields. Pass None for backward-compatible behavior.
  Also enhances color handling to accept 3‑digit hex, adds hex-bgr, and
  provides 16‑bit rgb16 plus normalized dec channel serialization.
• BREAKING: Using the CLI to sync with a custom --schemes-dir is not
  supported anymore. sync only syncs to the default
  $XDG_DATA_DIR/tinted-theming/tinted-builder-rust/schemes directory.
• Library: Extend Template to render Tinted8 schemes; update Base16/Base24
  code paths for consistency.
• CLI: Improve errors and messages around missing/invalid template config and
  tinted8 compatibility.

Fixed

• CLI: Add explicit error codes and validations for Tinted8 flows:
  E300/E301/E302 (missing supports entries), E303 (missing mustache),
  E305 (missing/invalid config), E400 (no schemes), plus E110/E111 for
  unknown systems/invalid files.

Removed

• BREAKING: Library: Remove list, listbase16, and listbase24 scheme
  systems and their SchemeSystem enum variants. SchemeSystem::from_str no
  longer accepts these values. Use the yaml config options.list flow for list
  generation instead.