Better & more decorations

[scoobynko]

Summary

Improve the existing ornaments and borders, and add more of both.

Current state

• Ornaments (src/welchost/ornaments.py): none | ghosts | stars | bars | dots — predefined ASCII blocks flanking the banner.
• Borders (decoration.border_style): panel | box | double | rounded | ascii | none.

Scope

• Add new ornament sets and richer border styles.
• Revisit the existing ones — the current sets were a first pass and some read better than others at different banner heights.
• Consider whether ornaments should adopt the banner palette the way the styled metadata row now does (info.accent inherits the banner color), rather than rendering unstyled.

Requirements

• Stay data-driven. New ornaments are plain data in ornaments.py, not rendering code. Same for border glyph sets. Adding an option should not mean touching the renderer.
• WYSIWYG in the preview. Every new option must render identically in the TUI BannerPreview and in the generated welcome_banner.py. These are two separate code paths (render.py vs. the Jinja2 template) and they drift silently — cover new options with a parity test like the one added for the metadata row.
• Width-aware. Ornaments flank the banner and count toward measured width; fit.art_width() already accounts for this, so new ornaments must feed the same measurement rather than bypassing it.
• Backward compatible. Existing welchost.toml files naming today's ornaments keep working, round-trip lossless.

Areas

Decoration, Rendering · Effort M