feat(PageHead): make page metadata site-aware with auto-derived canonical URLs

Extract pure resolvePageHeadMeta helper that derives title suffix,
canonical URL, and OpenGraph fallbacks from the active site config and
router pathname. PageHead now reads useSite() + useLocation() to
render per-subdomain metadata (e.g. " | Nessa", canonical on
nessa.freno.me) instead of hardcoding " | Michael Freno".

- Always emit a canonical link (explicit override wins; otherwise
  https://<site.domain><pathname>)
- Default og:image to the site ogDefaultImage when not provided
- Add unit tests covering all five sites, canonical derivation, and
  OpenGraph fallback behavior

src/components/page-head-meta.ts

@@ -0,0 +1,70 @@
+/**
+ * Pure metadata derivation for `PageHead` (task 02).
+ *
+ * Intentionally imports NOTHING from solid-js / @solidjs/router / @solidjs/meta
+ * so it can be unit-tested in `bun:test` without spinning up the SolidJS
+ * router + MetaProvider + DOM (which this repo does not configure). The
+ * `PageHead` component is a thin render layer over this function.
+ *
+ * Rules:
+ *  - `title` → `props.title + site.titleSuffix`
+ *  - `canonical` → explicit `props.canonical` override wins; otherwise
+ *    `https://${site.domain}${pathname}`. The pathname is the *browser* path
+ *    (from `useLocation`), which is correct because vercel.json host rewrites
+ *    target internal route prefixes (`/nessa`, `/lineage`, …) while leaving
+ *    the public URL intact — so `nessa.freno.me/contact` reports pathname
+ *    `/contact`, and the canonical is `https://nessa.freno.me/contact`.
+ *  - `ogImage` → explicit `props.ogImage` wins; otherwise `site.ogDefaultImage`.
+ *  - `ogTitle` / `ogDescription` → explicit override wins; otherwise fall
+ *    back to the base title (no suffix) / description (existing behavior).
+ */
+import type { Site } from "~/lib/site-context";
+
+export interface PageHeadProps {
+  title: string;
+  description?: string;
+  ogImage?: string;
+  ogTitle?: string;
+  ogDescription?: string;
+  canonical?: string;
+}
+
+/**
+ * Fully-resolved metadata computed by {@link resolvePageHeadMeta}. `PageHead`
+ * renders these verbatim. Kept as an exported type so call sites / tests can
+ * assert against the exact values without a DOM render.
+ */
+export interface ResolvedPageHeadMeta {
+  /** Page title with the active site's `titleSuffix` appended. */
+  title: string;
+  description?: string;
+  /** Canonical absolute URL for the current route. */
+  canonical: string;
+  /** OpenGraph title (falls back to the page title without suffix). */
+  ogTitle: string;
+  /** OpenGraph description (falls back to `description`). */
+  ogDescription?: string;
+  /** OpenGraph image (defaults to the site's `ogDefaultImage`). */
+  ogImage: string;
+}
+
+export function resolvePageHeadMeta(
+  props: PageHeadProps,
+  site: Site,
+  pathname: string
+): ResolvedPageHeadMeta {
+  const title = `${props.title}${site.titleSuffix}`;
+  const canonical = props.canonical ?? `https://${site.domain}${pathname}`;
+  const ogTitle = props.ogTitle ?? props.title;
+  const ogDescription = props.ogDescription ?? props.description;
+  const ogImage = props.ogImage ?? site.ogDefaultImage;
+
+  return {
+    title,
+    description: props.description,
+    canonical,
+    ogTitle,
+    ogDescription,
+    ogImage
+  };
+}