add tree-coloring appearance-tuning parameters

[jbloom]

Three new parameters that work alongside color_tree_by (released in the prior PR) to give users control over the tree-coloring legend and the scale-bar label:

• tree_color_scale (default None): override the default palette with an explicit {category: color} mapping. Keys must match the tree's categories one-to-one (mismatch raises a ValueError listing the actual categories so the user can copy them); legend order follows the dict's insertion order. "unknown" is auto-appended in gray and may not be specified by the user. CLI form is a comma-separated key=color list parsed by a custom click.ParamType.

• tree_color_legend_format (default None): pass any subset of Vega-Lite's Legend properties as a dict (orient, direction, columns, padding, labelFontSize, titleFontSize, …) shallow- merged onto the package defaults. Smart default: when the final orient is "left" or "right" and the user has not set columns or direction, columns=1 is forced — this counteracts chart-level legend.columns defaults that would otherwise pack a side-anchored legend horizontally. CLI form is a JSON object string.

• tree_color_legend_show (default True): explicit boolean to hide the legend while keeping the tree colored — replaces the older legend_loc="none" mechanism with a clearer surface.

• scale_bar_font_size (default 10): replaces the previously- hardcoded fontSize=10 in _build_scale_bar_layer.

The legend wiring lives entirely in _build_tree_chart: a default dict of {"orient": "bottom", "title": <derived>} is merged with the user's legend_format, then the smart-default columns rule is applied, then the result is passed to alt.Legend(...). The legend_show boolean determines whether alt.Color receives the built legend or legend=None.

CLI auto-generation in cli.py gains a second dict-typed param branch: _is_str_dict is tightened to match dict[str, str] exactly, and a new _is_dict_any predicate plus
_JsonDictParamType route dict[str, Any] (and its | None variant) to a JSON-object parser. The bool flag auto-flows through the existing dual-form handler (--tree-color-legend-show / --no-tree-color-legend-show).

Documentation: a new "Customizing the colors and legend" subsection in docs/examples.md shows all four knobs with a worked H3N2 example (Okabe-Ito-inspired 6-color palette ordered K, J.2.4, J.2.3, J.2.2, J.2, G.1.3.1; legend on the left at 14pt; scale bar at 14pt). The scripts/generate_docs_assets.py build adds an h3n2_combined_custom_colors SVG + interactive HTML that the section embeds and links.

Tests: tests/test_appearance_tuning.py (new, 27 tests) covers scale validation (order preservation, missing/extra/unknown keys, genotype/haplotype category-string formats); plot() end-to-end (scale propagation, scale-without-color_tree_by raises); legend format defaults, font-size propagation, orient override, smart-default columns=1 for left/right, user-columns and user-direction beating the smart default, smart default skipped for top/bottom orients; legend_show hide; scale-bar font size; and CLI coverage for all four knobs (including JSON-error paths).