feat: add data contract validation

JSON Schemas for the database, install and pack manifests, target
manifests, site API envelopes and stats, plus the semantic invariants a
schema cannot express: declared totals matching their lists, no
destination both installed and omitted, database keys matching their
sha1. validate_site.py checks the rendered HTML for metadata, headings,
image alternatives, duplicate ids and unresolved local links.

Pack manifests are read from inside the generated archives, where
generate_pack writes them, rather than from a dist/ glob that matches
nothing.

Emulator and platform schemas gain additionalProperties: false, and
cores[] plus contents[].name must be strings: an unquoted 81 or 01 in
YAML parses as a number and stops matching the upstream name.

tests/test_site_validation.py

@@ -0,0 +1,96 @@
+"""Tests for rendered-site metadata and link validation."""
+
+from __future__ import annotations
+
+import json
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+TMP_ROOT = ROOT / "tmp" / "tests"
+TMP_ROOT.mkdir(parents=True, exist_ok=True)
+sys.path.insert(0, str(ROOT / "scripts"))
+
+from validate_site import validate_site  # noqa: E402
+
+
+def _page(title: str, description: str, body: str = "") -> str:
+    structured = json.dumps(
+        {"@context": "https://schema.org", "@type": "TechArticle"}
+    )
+    return f"""<!doctype html>
+<html lang="en">
+<head>
+  <title>{title}</title>
+  <meta name="description" content="{description}">
+  <link rel="canonical" href="https://example.test/retrobios/">
+  <script type="application/ld+json">{structured}</script>
+</head>
+<body><main><h1 id="top">{title}</h1>{body}</main></body>
+</html>
+"""
+
+
+class RenderedSiteValidation(unittest.TestCase):
+    def setUp(self) -> None:
+        self.temp = tempfile.TemporaryDirectory(dir=TMP_ROOT)
+        self.root = Path(self.temp.name)
+        self.site = self.root / "site"
+        self.site.mkdir()
+        self.config = self.root / "mkdocs.yml"
+        self.config.write_text(
+            "site_url: https://example.test/retrobios/\n", encoding="utf-8"
+        )
+
+    def tearDown(self) -> None:
+        self.temp.cleanup()
+
+    def test_valid_page_and_base_path_link_pass(self):
+        asset = self.site / "assets" / "file.json"
+        asset.parent.mkdir()
+        asset.write_text("{}\n", encoding="utf-8")
+        (self.site / "index.html").write_text(
+            _page(
+                "Home",
+                "Unique home description.",
+                '<a href="/retrobios/#top">Top</a>'
+                '<a href="/retrobios/assets/file.json">Data</a>'
+                '<img src="assets/logo.png" alt="Logo">',
+            ),
+            encoding="utf-8",
+        )
+        (self.site / "assets" / "logo.png").write_bytes(b"png")
+        self.assertEqual(validate_site(self.site, self.config), [])
+
+    def test_broken_fragment_and_missing_alt_fail(self):
+        (self.site / "index.html").write_text(
+            _page(
+                "Home",
+                "Unique home description.",
+                '<a href="#absent">Missing</a><img src="missing.png">',
+            ),
+            encoding="utf-8",
+        )
+        issues = validate_site(self.site, self.config)
+        self.assertTrue(any("missing fragment" in issue for issue in issues))
+        self.assertTrue(any("images missing alt" in issue for issue in issues))
+        self.assertTrue(any("broken local link" in issue for issue in issues))
+
+    def test_duplicate_search_metadata_fails(self):
+        (self.site / "index.html").write_text(
+            _page("Repeated", "Repeated description."), encoding="utf-8"
+        )
+        child = self.site / "child"
+        child.mkdir()
+        (child / "index.html").write_text(
+            _page("Repeated", "Repeated description."), encoding="utf-8"
+        )
+        issues = validate_site(self.site, self.config)
+        self.assertTrue(any("duplicate title" in issue for issue in issues))
+        self.assertTrue(any("duplicate description" in issue for issue in issues))
+
+
+if __name__ == "__main__":
+    unittest.main()