Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/preflight/version_test.go

@@ -0,0 +1,58 @@
+package preflight
+
+import "testing"
+
+func TestParseSemver(t *testing.T) {
+	cases := []struct {
+		in      string
+		want    string
+		wantErr bool
+	}{
+		{"stalwart 0.15.5\n", "0.15.5", false},
+		{"v0.16.14", "0.16.14", false},
+		{"Stalwart Mail Server v0.16.0 (build abc123)", "0.16.0", false},
+		{"no version here", "", true},
+		{"", "", true},
+	}
+	for _, tc := range cases {
+		got, err := parseSemver(tc.in)
+		if tc.wantErr {
+			if err == nil {
+				t.Errorf("parseSemver(%q) = %v, want error", tc.in, got)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("parseSemver(%q) unexpected error: %v", tc.in, err)
+			continue
+		}
+		if got.String() != tc.want {
+			t.Errorf("parseSemver(%q) = %s, want %s", tc.in, got, tc.want)
+		}
+	}
+}
+
+func TestSemverCompare(t *testing.T) {
+	cases := []struct {
+		a, b string
+		want int
+	}{
+		{"0.15.5", "0.16.0", -1},
+		{"0.16.14", "0.16.14", 0},
+		{"0.16.1", "0.15.5", 1},
+		{"1.0.0", "0.16.14", 1},
+	}
+	for _, tc := range cases {
+		a, err := parseSemver(tc.a)
+		if err != nil {
+			t.Fatalf("parseSemver(%q): %v", tc.a, err)
+		}
+		b, err := parseSemver(tc.b)
+		if err != nil {
+			t.Fatalf("parseSemver(%q): %v", tc.b, err)
+		}
+		if got := a.Compare(b); got != tc.want {
+			t.Errorf("%s.Compare(%s) = %d, want %d", tc.a, tc.b, got, tc.want)
+		}
+	}
+}