Filter sitemap by excluded/draft/noindex

src/routes/sitemap-appendix.xml/+server.js enumerates every +page.svelte from getPages() without honoring the excluded, draft, or noindex flags. Engineering todos like fix-ci-type-errors and migrate-to-lightning-css get published to search engines as article content.

Current State

• sitemap-appendix.xml/+server.js:46-52 — no filter at all
• sitemap/+page.server.js:73 — excludes only the sitemap page itself
• pages.remote.js getReadNext:50 — filters both excluded and draft

Tasks

• Centralize inclusion policy in a single helper (e.g. isPublic(page))
• Honor excluded, draft, noindex
• Apply the helper in all three consumers