Add install script, release Makefile and self-update

Releases are built locally (no runner) and uploaded to the Gitea generic
package registry. install.sh installs submarine plus a sub symlink;
`submarine update` replaces the binary after verifying its checksum.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/update/update_test.go

@@ -0,0 +1,106 @@
+package update
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestNewer(t *testing.T) {
+	cases := []struct {
+		cur, latest string
+		want        bool
+	}{
+		{"v0.1.0", "v0.2.0", true},
+		{"v0.2.0", "v0.2.0", false},
+		{"v0.10.0", "v0.9.0", false},
+		{"v1.0.0", "v0.9.9", false},
+		{"dev", "v0.1.0", true},
+		{"v0.1.0-3-gabc", "v0.1.0", true},
+		{"v0.1.0", "garbage", false},
+	}
+	for _, c := range cases {
+		if got := Newer(c.cur, c.latest); got != c.want {
+			t.Errorf("Newer(%q, %q) = %v, want %v", c.cur, c.latest, got, c.want)
+		}
+	}
+}
+
+func registry(t *testing.T, bin, sum string) *Client {
+	t.Helper()
+	mux := http.NewServeMux()
+	mux.HandleFunc("/submarine-latest/latest/VERSION", func(w http.ResponseWriter, _ *http.Request) {
+		w.Write([]byte("v0.2.0\n"))
+	})
+	mux.HandleFunc("/submarine/v0.2.0/checksums.txt", func(w http.ResponseWriter, _ *http.Request) {
+		w.Write([]byte(sum + "  submarine_linux_arm64\n"))
+	})
+	mux.HandleFunc("/submarine/v0.2.0/submarine_linux_arm64", func(w http.ResponseWriter, _ *http.Request) {
+		w.Write([]byte(bin))
+	})
+	srv := httptest.NewServer(mux)
+	t.Cleanup(srv.Close)
+	return &Client{BaseURL: srv.URL, HTTP: srv.Client(), GOOS: "linux", GOARCH: "arm64"}
+}
+
+func sha(s string) string { h := sha256.Sum256([]byte(s)); return hex.EncodeToString(h[:]) }
+
+func TestLatest(t *testing.T) {
+	c := registry(t, "x", sha("x"))
+	v, err := c.Latest(context.Background())
+	if err != nil || v != "v0.2.0" {
+		t.Fatalf("Latest = %q, %v", v, err)
+	}
+}
+
+func TestInstallReplaces(t *testing.T) {
+	dir := t.TempDir()
+	target := filepath.Join(dir, "submarine")
+	os.WriteFile(target, []byte("old"), 0o755)
+	c := registry(t, "new-binary", sha("new-binary"))
+	if err := c.InstallTo(context.Background(), "v0.2.0", target); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := os.ReadFile(target)
+	if string(got) != "new-binary" {
+		t.Fatalf("target = %q", got)
+	}
+	if fi, _ := os.Stat(target); fi.Mode().Perm()&0o111 == 0 {
+		t.Error("target not executable")
+	}
+	if entries, _ := os.ReadDir(dir); len(entries) != 1 {
+		t.Errorf("leftover files: %v", entries)
+	}
+}
+
+func TestInstallChecksumMismatch(t *testing.T) {
+	dir := t.TempDir()
+	target := filepath.Join(dir, "submarine")
+	os.WriteFile(target, []byte("old"), 0o755)
+	c := registry(t, "tampered", sha("original"))
+	err := c.InstallTo(context.Background(), "v0.2.0", target)
+	if err == nil || !strings.Contains(err.Error(), "checksum mismatch") {
+		t.Fatalf("err = %v", err)
+	}
+	if got, _ := os.ReadFile(target); string(got) != "old" {
+		t.Errorf("target modified: %q", got)
+	}
+	if entries, _ := os.ReadDir(dir); len(entries) != 1 {
+		t.Errorf("temp file left behind: %v", entries)
+	}
+}
+
+func TestInstallMissingAsset(t *testing.T) {
+	c := registry(t, "x", sha("x"))
+	c.GOARCH = "riscv64"
+	err := c.InstallTo(context.Background(), "v0.2.0", filepath.Join(t.TempDir(), "submarine"))
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}