feat(i18n): English localization (phase 3)

- Translate content to English across messages/en.json, data/en, lib/data/en, and public/exemplos/en
- Implement localized routing, slugs, and language negotiation with fallback to en
- Build dedicated search index for English with strict book exclusion
- Update components and pages to dynamically render localized strings and datasets
- Verify full structural parity and exact 1:1 match on Portuguese pages via comparator

Co-Authored-By: Antigravity <noreply@google.com>

lib/seo.js

@@ -6,6 +6,8 @@
  * nascendo sem título próprio.
  */
 
+import { ENABLED_LOCALES, getLocalizedHref } from "@/i18n/links";
+
 export const SITE = {
 	url: "https://espdocs.cienciaembarcada.com.br",
 	nome: "ESPDocs",
@@ -28,16 +30,49 @@ export const SITE = {
  * Usar isto em vez de escrever o objeto à mão garante que nenhuma página fique
  * sem canonical ou sem og:title.
  */
-export function paginaMeta({ titulo, descricao, caminho, keywords = [], tipo = "website" }) {
-	const url = `${SITE.url}${caminho}`;
+export function paginaMeta({ titulo, descricao, caminho, keywords = [], tipo = "website", locale = "pt" }) {
+	const currentLocale = locale || "pt";
+	const basePath = caminho.startsWith("/") ? caminho : `/${caminho}`;
+	const canonicalPath = getLocalizedHref(`/pt${basePath}`, currentLocale);
+	const url = `${SITE.url}${canonicalPath}`;
+
+	const languages = {};
+	for (const loc of ENABLED_LOCALES) {
+		if (basePath === "/livro" && loc !== "pt") continue;
+		const locHref = getLocalizedHref(`/pt${basePath}`, loc);
+		const langKey = loc === "pt" ? "pt-BR" : loc;
+		languages[langKey] = `${SITE.url}${locHref}`;
+	}
+	if (ENABLED_LOCALES.includes("en") && basePath !== "/livro") {
+		languages["x-default"] = `${SITE.url}${getLocalizedHref(`/pt${basePath}`, "en")}`;
+	}
+
+	const ogLocaleMap = {
+		pt: "pt_BR",
+		en: "en_US",
+		es: "es_419",
+		fr: "fr_FR",
+		de: "de_DE",
+		"zh-CN": "zh_CN",
+	};
+
+	const alternateLocales = ENABLED_LOCALES
+		.filter((l) => l !== currentLocale && (basePath !== "/livro" || l === "pt"))
+		.map((l) => ogLocaleMap[l])
+		.filter(Boolean);
+
 	return {
 		title: titulo,
 		description: descricao,
 		keywords,
-		alternates: { canonical: caminho },
+		alternates: {
+			canonical: canonicalPath,
+			languages: Object.keys(languages).length > 1 ? languages : undefined,
+		},
 		openGraph: {
 			type: tipo,
-			locale: "pt_BR",
+			locale: ogLocaleMap[currentLocale] || "pt_BR",
+			alternateLocale: alternateLocales.length > 0 ? alternateLocales : undefined,
 			url,
 			siteName: SITE.nome,
 			title: `${titulo} · ${SITE.nome}`,