refactor: move coloring logic into coloring.rs, tighten visibility

[Copilot]

Closes #343.

Sub-PR targeting copilot/add-color-flag-and-mapping (#339). Resolves the five TODO items left open in that PR.

Changes

src/visualizer/coloring.rs

• Added Coloring::colored_slice — combines both TODO blocks from methods.rs into a single method that accepts a path-components iterator, has_children, and a TreeHorizontalSlice<String>, returning MaybeColoredTreeHorizontalSlice<'a>
• Coloring::node_color: pub(super) → private (only called by colored_slice)
• Color::ansi_prefix: pub(super) → private
• AnsiPrefix: pub(super) struct → struct
• ColoredTreeHorizontalSlice fields: pub(super) → private (struct constructed exclusively inside colored_slice)
• Removed all three TODO comments

src/visualizer/methods.rs

• Replaced the two TODO blocks (14 lines) with a match self.coloring that delegates to coloring.colored_slice(...)
• Dropped unused imports: ColoredTreeHorizontalSlice, pipe_trait::Pipe (replaced pipe_as_ref(once) with std::iter::once)

// Before: two separate TODO-marked blocks
let colored = self.coloring.and_then(|coloring| { /* build path, call node_color */ });
let tree = match colored { Some((color, ls_colors)) => Colorful(...), None => Colorless(...) };

// After: single delegating call
let tree = match self.coloring {
    Some(coloring) => coloring.colored_slice(path_components, has_children, slice),
    None => MaybeColoredTreeHorizontalSlice::Colorless(tree_horizontal_slice),
};

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[KSXGitHub]

Superseded by #343.