Add Lit example implementation

[oddcelot]

Summary

• Add a Lit 3 micro example demonstrating Texivia routing (locale-prefixed routes, parameterized paths, redirects, layout composition, 404 fallback).
• Document the Lit integration in the root README.md with a worked <app-shell> snippet and the Light DOM constraint that makes click interception work.
• Emit dist/texivia.d.ts from the publish build so TypeScript users get types out of the box.
• Add lit and web-components to package keywords; add Lit to the framework support matrix.

Implementation notes

• Router shape. Router<View> is parameterized over a render function: View = (params) => TemplateResult. Each route's view is the render fn the example swaps into a single root <app-shell>.
• Light DOM is load-bearing, not stylistic. Texivia's click handler calls event.target.closest('a'). Shadow DOM retargets event.target to the host (<app-shell>), so closest('a') walks past the actual anchor. The example uses createRenderRoot() { return this } to keep <a> as the literal target. The README and in-code comments both spell this out.
• Listener before start(). Routes without a handler dispatch the texivia event synchronously inside router.start(), so a late listener misses the initial event on deep-link loads.
• Layouts as plain functions. mainLayout(locale, body) is a template function, not a LitElement — avoids <slot> ceremony in light DOM and adds no state of its own.
• Initial locale seed. <app-shell> seeds params.locale from navigator.language.split('-')[0] so the first paint matches the '/' → '/<lang>/' redirect, eliminating a flash of wrong-locale links.
• Login form is inert. @submit=${e => e.preventDefault()} instead of a real POST, since the example demos routing, not auth.