coverage: make targets + close zero-cov gaps (namegen, sessionstream)

Adds `make coverage` (per-package + total via -coverpkg=./...),
`make coverage-html`, and `make coverage-total` (CI-friendly). Wires
coverage.out/coverage.html through `make clean` and .gitignore.

Closes the two easy zero-coverage packages: namegen (77.8%) and
sessionstream (93.5%). Total statement coverage 51.7% -> 52.1%.

internal/namegen/namegen_test.go

@@ -0,0 +1,54 @@
+package namegen
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestGenerate(t *testing.T) {
+	adjSet := make(map[string]struct{}, len(adjectives))
+	for _, a := range adjectives {
+		adjSet[a] = struct{}{}
+	}
+	subSet := make(map[string]struct{}, len(substantives))
+	for _, s := range substantives {
+		subSet[s] = struct{}{}
+	}
+
+	seen := make(map[string]int)
+	for i := 0; i < 200; i++ {
+		name := Generate()
+		parts := strings.Split(name, "-")
+		if len(parts) != 2 {
+			t.Fatalf("expected adj-noun form, got %q", name)
+		}
+		if _, ok := adjSet[parts[0]]; !ok {
+			t.Fatalf("unknown adjective %q in %q", parts[0], name)
+		}
+		if _, ok := subSet[parts[1]]; !ok {
+			t.Fatalf("unknown substantive %q in %q", parts[1], name)
+		}
+		seen[name]++
+	}
+
+	// Minimal variety check: adj-noun cartesian product is thousands of
+	// combinations; 200 draws should hit more than a couple.
+	if len(seen) < 10 {
+		t.Fatalf("expected varied output, only saw %d distinct names", len(seen))
+	}
+}
+
+func TestRandomIndex(t *testing.T) {
+	if got := randomIndex(0); got != 0 {
+		t.Fatalf("randomIndex(0) = %d, want 0", got)
+	}
+	if got := randomIndex(1); got != 0 {
+		t.Fatalf("randomIndex(1) = %d, want 0", got)
+	}
+	for i := 0; i < 100; i++ {
+		n := randomIndex(7)
+		if n < 0 || n >= 7 {
+			t.Fatalf("randomIndex(7) = %d, out of range", n)
+		}
+	}
+}