embed blogs.json instead of fetching it at runtime (closes #1)

The library needed network access on first use and its results changed
under the caller between runs. blogs.json is now vendored and compiled in
with go:embed, so the dataset is fixed for a given build.

FetchBlogs keeps its name, signature and sync.Once memoization but now
decodes the embedded bytes; its error return is only reachable if the
committed blogs.json is malformed. net/http is gone from the package, and
a test asserts no net/* import returns to non-test code.

make update-data refreshes the vendored file, reading the upstream
location from the BlogsURL constant so the URL has one definition. It
downloads to a temporary file, replaces blogs.json only on a complete
download, and runs the test suite against the new data.

The dataset is committed verbatim as upstream serves it, which is ~8 MB
of JSON in the repo and in every linking binary; most of that is per-blog
post history that the Blog struct does not expose.

Model: opus-5

hnblogs_test.go

@@ -1,6 +1,9 @@
 package hnblogs
 
 import (
+	"encoding/json"
+	"go/build"
+	"strings"
 	"testing"
 )
 
@@ -11,18 +14,63 @@ func TestFetchBlogs(t *testing.T) {
 	}
 
 	if len(blogs) == 0 {
-		t.Fatalf("Expected to fetch some blogs, got %d", len(blogs))
+		t.Fatalf("Expected the embedded dataset to be non-empty, got %d blogs", len(blogs))
 	}
 }
 
-func TestGetBlogs(t *testing.T) {
+// TestEmbeddedDataIsUsable guards the invariant the embedded dataset has to
+// satisfy for the rest of the package: every entry has a URL, so callers of
+// RandomBlog and NthBlog get something dereferenceable.
+func TestEmbeddedDataIsUsable(t *testing.T) {
+	if !json.Valid(blogsJSON) {
+		t.Fatalf("Embedded blogs.json is not valid JSON")
+	}
+
 	blogs, err := GetBlogs()
 	if err != nil {
 		t.Fatalf("Expected no error, got %v", err)
 	}
 
-	if len(blogs) == 0 {
-		t.Fatalf("Expected to fetch some blogs, got %d", len(blogs))
+	for i, blog := range blogs {
+		if blog.URL == "" {
+			t.Fatalf("Blog at index %d has an empty URL", i)
+		}
+	}
+}
+
+// TestNoRuntimeNetworkImports is the regression guard for the reason this data
+// is embedded: the package must not reach the network on any code path. A
+// transport dependency reintroduced in non-test code fails here.
+func TestNoRuntimeNetworkImports(t *testing.T) {
+	pkg, err := build.ImportDir(".", 0)
+	if err != nil {
+		t.Fatalf("Failed to inspect package imports: %v", err)
+	}
+
+	for _, imported := range pkg.Imports {
+		if imported == "net" || strings.HasPrefix(imported, "net/") {
+			t.Fatalf("Package must not perform network I/O, but imports %q", imported)
+		}
+	}
+}
+
+func TestGetBlogsIsMemoized(t *testing.T) {
+	first, err := GetBlogs()
+	if err != nil {
+		t.Fatalf("Expected no error, got %v", err)
+	}
+
+	second, err := GetBlogs()
+	if err != nil {
+		t.Fatalf("Expected no error, got %v", err)
+	}
+
+	if len(first) != len(second) {
+		t.Fatalf("Expected the same slice on repeated calls, got %d then %d", len(first), len(second))
+	}
+
+	if len(first) > 0 && &first[0] != &second[0] {
+		t.Fatalf("Expected repeated calls to share the memoized backing array")
 	}
 }
 
@@ -55,6 +103,19 @@ func TestRandomBlogs(t *testing.T) {
 	}
 }
 
+func TestRandomBlogsRejectsInvalidCounts(t *testing.T) {
+	blogs, err := GetBlogs()
+	if err != nil {
+		t.Fatalf("Expected no error, got %v", err)
+	}
+
+	for _, n := range []int{0, -1, len(blogs) + 1} {
+		if _, err := RandomBlogs(n); err == nil {
+			t.Fatalf("Expected an error for n=%d, got none", n)
+		}
+	}
+}
+
 func TestNthBlog(t *testing.T) {
 	blogs, err := GetBlogs()
 	if err != nil {
@@ -74,4 +135,8 @@ func TestNthBlog(t *testing.T) {
 	if err == nil {
 		t.Fatalf("Expected error for out-of-range index, got none")
 	}
+
+	if _, err := NthBlog(-1); err == nil {
+		t.Fatalf("Expected error for negative index, got none")
+	}
 }