Add canonical URLs and sitemap dedup for docs homepage indexability

- Emit self-referential canonical `<link>` tags on every VitePress page
- Filter duplicate /README entry from generated sitemap
- Extract DOCS_HOSTNAME constant; update Plausible test to match
- Add seo.test.ts covering canonical generation and sitemap filtering

docs-site/seo.test.ts

@@ -0,0 +1,40 @@
+import { expect, test } from 'bun:test';
+import type { TransformContext } from 'vitepress';
+import docsConfig from './.vitepress/config';
+
+function makeTransformContext(page: string): TransformContext {
+  return {
+    page,
+    siteConfig: {} as TransformContext['siteConfig'],
+    siteData: {} as TransformContext['siteData'],
+    pageData: {} as TransformContext['pageData'],
+    title: 'SubMiner',
+    description: 'SubMiner docs',
+    head: [],
+    content: '',
+    assets: [],
+  };
+}
+
+test('docs pages emit stable self-referential canonical URLs', async () => {
+  const rootHead = await docsConfig.transformHead?.(makeTransformContext('index.md'));
+  const usageHead = await docsConfig.transformHead?.(makeTransformContext('usage.md'));
+
+  expect(rootHead).toContainEqual([
+    'link',
+    { rel: 'canonical', href: 'https://docs.subminer.moe/' },
+  ]);
+  expect(usageHead).toContainEqual([
+    'link',
+    { rel: 'canonical', href: 'https://docs.subminer.moe/usage' },
+  ]);
+  expect(JSON.stringify(rootHead).toLowerCase()).not.toContain('noindex');
+});
+
+test('docs sitemap excludes duplicate README page from indexable URLs', async () => {
+  const items = [{ url: '' }, { url: 'README' }, { url: 'usage' }];
+
+  const transformedItems = await docsConfig.sitemap?.transformItems?.(items);
+
+  expect(transformedItems?.map((item) => item.url)).toEqual(['', 'usage']);
+});