feat: generate per-subdomain sitemaps based on Host header

Refactor sitemap.xml route to read the Host header, determine the active
site, and produce a sitemap scoped to that site's routes with canonical
URLs. Extracts route definitions and XML generation into shared modules.

src/lib/sitemap-routes.ts

@@ -0,0 +1,74 @@
+/**
+ * Centralized sitemap route registry per site.
+ *
+ * Each entry describes a URL path that should appear in the corresponding
+ * site's `sitemap.xml`, along with SEO metadata (change frequency and
+ * relative priority).
+ *
+ * This module is pure — no imports of server-side code — so it can be
+ * used in unit tests and from both server and client contexts.
+ */
+import type { SiteId } from "./site-context";
+
+export interface SitemapEntry {
+  /**
+   * Absolute path on the site's domain (must start with `/`).
+   */
+  path: string;
+
+  /**
+   * Expected change frequency.
+   */
+  changefreq: "always" | "hourly" | "daily" | "weekly" | "monthly" | "yearly" | "never";
+
+  /**
+   * Relative priority (0.0–1.0).
+   */
+  priority: number;
+}
+
+/**
+ * Per-site sitemap route definitions.
+ *
+ * Entries for subdomain pages (contact, privacy, downloads, etc.) are
+ * populated as those pages are built in tasks 05–11.
+ */
+export const SITEMAP_ROUTES: Record<SiteId, SitemapEntry[]> = {
+  main: [
+    { path: "/", changefreq: "weekly", priority: 1.0 },
+    { path: "/blog", changefreq: "daily", priority: 0.9 },
+    { path: "/contact", changefreq: "monthly", priority: 0.7 },
+    { path: "/login", changefreq: "monthly", priority: 0.5 },
+    { path: "/resume", changefreq: "yearly", priority: 0.6 },
+    { path: "/downloads", changefreq: "weekly", priority: 0.8 }
+  ],
+
+  // ── Subdomain sites ──────────────────────────────────────────────────
+  // Populated as pages land in tasks 05–11.
+
+  nessa: [
+    { path: "/", changefreq: "weekly", priority: 1.0 },
+    { path: "/contact", changefreq: "monthly", priority: 0.6 },
+    { path: "/privacy", changefreq: "yearly", priority: 0.4 }
+  ],
+
+  lineage: [
+    { path: "/", changefreq: "weekly", priority: 1.0 },
+    { path: "/contact", changefreq: "monthly", priority: 0.6 },
+    { path: "/privacy", changefreq: "yearly", priority: 0.4 },
+    { path: "/downloads", changefreq: "weekly", priority: 0.8 },
+    { path: "/deletion", changefreq: "yearly", priority: 0.3 }
+  ],
+
+  gaze: [
+    { path: "/", changefreq: "weekly", priority: 1.0 },
+    { path: "/contact", changefreq: "monthly", priority: 0.6 },
+    { path: "/privacy", changefreq: "yearly", priority: 0.4 }
+  ],
+
+  inputhalo: [
+    { path: "/", changefreq: "weekly", priority: 1.0 },
+    { path: "/contact", changefreq: "monthly", priority: 0.6 },
+    { path: "/privacy", changefreq: "yearly", priority: 0.4 }
+  ]
+};