feat(cli): real doctor subcommand with adb/emulator/java checks

Each check is a value so tests can swap in fakes. Java check parses
both legacy (1.8) and modern (17+) version strings. Emulator check
falls back to ANDROID_HOME/emulator/emulator since the brew cask
ships it without putting it on PATH.

cmd/uatu/doctor.go

@@ -0,0 +1,119 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"time"
+)
+
+type doctorCheck struct {
+	Name string
+	Run  func(ctx context.Context) error
+}
+
+func defaultDoctorChecks() []doctorCheck {
+	return []doctorCheck{
+		{Name: "adb on PATH", Run: checkExecutableOnPath("adb")},
+		{Name: "emulator on PATH or under ANDROID_HOME", Run: checkEmulator},
+		{Name: "java 17+ on PATH", Run: checkJavaVersion},
+	}
+}
+
+func runDoctorChecks(ctx context.Context, checks []doctorCheck, stdout io.Writer) error {
+	failures := 0
+	for _, check := range checks {
+		callCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+		err := check.Run(callCtx)
+		cancel()
+		if err != nil {
+			fmt.Fprintf(stdout, "FAIL  %s: %v\n", check.Name, err)
+			failures++
+			continue
+		}
+		fmt.Fprintf(stdout, "OK    %s\n", check.Name)
+	}
+	if failures > 0 {
+		return fmt.Errorf("%d check(s) failed", failures)
+	}
+	return nil
+}
+
+func checkExecutableOnPath(name string) func(context.Context) error {
+	return func(_ context.Context) error {
+		if _, err := exec.LookPath(name); err != nil {
+			return fmt.Errorf("not found: %w", err)
+		}
+		return nil
+	}
+}
+
+func checkEmulator(_ context.Context) error {
+	if _, err := exec.LookPath("emulator"); err == nil {
+		return nil
+	}
+	androidHome := os.Getenv("ANDROID_HOME")
+	if androidHome == "" {
+		androidHome = os.Getenv("ANDROID_SDK_ROOT")
+	}
+	if androidHome == "" {
+		return fmt.Errorf("not on PATH and ANDROID_HOME is unset")
+	}
+	candidate := filepath.Join(androidHome, "emulator", "emulator")
+	if _, err := os.Stat(candidate); err != nil {
+		return fmt.Errorf("not found at %s", candidate)
+	}
+	return nil
+}
+
+var javaVersionPattern = regexp.MustCompile(`(?:java|openjdk)[^"]*"(\d+)(?:\.(\d+))?`)
+
+func checkJavaVersion(ctx context.Context) error {
+	if _, err := exec.LookPath("java"); err != nil {
+		return fmt.Errorf("java not found: %w", err)
+	}
+	output, err := exec.CommandContext(ctx, "java", "-version").CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("java -version: %w", err)
+	}
+	major, err := parseJavaMajor(string(output))
+	if err != nil {
+		return err
+	}
+	if major < 17 {
+		return fmt.Errorf("java major version %d is less than 17", major)
+	}
+	return nil
+}
+
+func parseJavaMajor(versionOutput string) (int, error) {
+	match := javaVersionPattern.FindStringSubmatch(versionOutput)
+	if match == nil {
+		return 0, fmt.Errorf("could not parse java version from %q", firstLine(versionOutput))
+	}
+	major, err := strconv.Atoi(match[1])
+	if err != nil {
+		return 0, fmt.Errorf("non-numeric major %q", match[1])
+	}
+	if major == 1 && len(match) >= 3 && match[2] != "" {
+		minor, err := strconv.Atoi(match[2])
+		if err == nil {
+			return minor, nil
+		}
+	}
+	return major, nil
+}
+
+func firstLine(text string) string {
+	for index := 0; index < len(text); index++ {
+		if text[index] == '\n' {
+			return text[:index]
+		}
+	}
+	return text
+}