fix(deps): use iced_core for color conversions

[JosephDemarest]

Summary

• replace the optional full iced dependency with iced_core, which owns the converted Color and Background types
• keep full iced as a dev dependency so the existing GUI example still compiles and exercises the conversions
• preserve the public iced feature while avoiding renderer, windowing, and executor defaults for downstream users

The normal/build dependency graph for --no-default-features --features iced drops from 206 unique packages to 33 (173 fewer, 83.98%).

Closes #77

Verification

• cargo check --no-default-features --features iced --lib
• cargo check --example iced --features iced
• cargo build --features "ansi-term bevy css-colors iced ratatui serde" --verbose
• cargo test --features "ansi-term bevy css-colors iced ratatui serde" --verbose
• cargo fmt --all --check
• cargo check --all-targets --all-features
• cargo package --allow-dirty

uvx pre-commit run --all-files reaches an existing Rust 1.97 Clippy failure in build.rs:212 (iter_kv_map), reproduced unchanged on origin/main. All other hooks pass for the changed files.