Add title, facts and notice rows

Header draws only the rows it is given, top to bottom:

Row	Turn it on with
A title rule, the name in the middle, a slot each side	title(Some(..)), left(..), right(..)
The groups and sections rows	on by default; tabs(false) leaves them out
A facts row, plain or on a rule	facts(&[..]), facts_rule(true)
A breadcrumb row	crumbs(true); see Drill in with a breadcrumb
A closing rule	closing(true)
A notice row, for a result, a warning or a question	notice(Some(..))

height() answers how many rows that makes, so the app lays out the rest of the frame under it. This is the header in the image above, from examples/gallery.rs rows:

use pito_header::{Fact, Group, Header, Nav, Section, Styles};
use ratatui::{Frame, layout::Rect, style::{Color, Modifier, Style}};

const ACCENT: Style = Style::new().fg(Color::Yellow);
const MUTED: Style = Style::new().fg(Color::DarkGray);
const STYLES: Styles = Styles::new().accent(ACCENT).muted(MUTED).rule(MUTED);

fn draw(frame: &mut Frame) {
    let mut nav = Nav::new(vec![
        Group::new("Mail").section(Section::new("Inbox")).section(Section::new("Drafts")),
        Group::new("Work").section(Section::new("Checks")),
    ]);
    nav.go(2);
    let trial = [Fact::new("trial ", MUTED), Fact::new("9 days left", ACCENT)];
    let facts = [
        Fact::new("3 drafts", ACCENT),
        Fact::new("1 scheduled", MUTED),
        Fact::new("synced just now", MUTED),
    ];
    let header = Header::new(&nav)
        .styles(STYLES)
        .title(Some("Desk"))
        .left_parts(&trial)
        .right(Some(Fact::new("? help", MUTED)))
        .facts(&facts)
        .facts_rule(true)
        .closing(true)
        .notice(Some(Fact::new(
            "Delete the 3 drafts? y/n",
            Style::new().fg(Color::Magenta).add_modifier(Modifier::BOLD),
        )));
    let area = frame.area();
    frame.render_widget(header, Rect { height: header.height().min(area.height), ..area });
}

Styles

Styles holds three styles: accent for the current group and section (drawn bold), muted for the rest of the nav, and rule for the rules. Every Fact carries its own style, and text on a rule carries exactly that style: the rule’s style never leaks into it. With no styles(..), everything draws in the terminal’s default style.

Slots and facts

• The side slots sit on the title rule, so they show only with a title, which draws in the accent, bold.
• A slot holds one Fact (left, right), several side by side in their own styles (left_parts, right_parts), or strings the app owns (left_pairs, right_pairs, from a slice of (String, Style)), so the app needs no Vec<Fact> built each frame.
• facts and facts_pairs take the facts row the same two ways. Facts are joined by ·, or by what separator(..) sets.
• Empty facts are skipped, and a row of only empty facts takes no row. An empty title is no title.
• When the width runs out, the facts row elides with “…” only when the room left can’t hold the separator and the next fact’s first two cells, and a cut fact ends the row. When a slot is cut, the “…” takes the style of the part it cuts. The side slots drop before the title does.