Add testable CLI with dependency injection and new scanner/checker packages

Major changes:
- Refactor CLI to accept injected I/O streams and filesystem (afero.Fs)
  for testing without touching the real filesystem
- Add RunOptions struct and RunWithOptions() for configurable CLI execution
- Add internal/scanner package with two-phase manifest generation:
  - Phase 1 (Enumeration): walk directories, collect metadata
  - Phase 2 (Scan): read contents, compute hashes, write manifest
- Add internal/checker package for manifest verification with progress
  reporting and channel-based result streaming
- Add mfer/builder.go for incremental manifest construction
- Add --no-extra-files flag to check command to detect files not in manifest
- Add timing summaries showing file count, size, elapsed time, and throughput
- Add comprehensive tests using afero.MemMapFs (no real filesystem access)
- Add contrib/usage.sh integration test script
- Fix banner ASCII art alignment (consistent spacing)
- Fix verbosity levels so summaries display at default log level
- Update internal/log to support configurable output writers

internal/log/log.go

@@ -2,7 +2,10 @@ package log
 
 import (
 	"fmt"
+	"io"
+	"os"
 	"runtime"
+	"sync"
 
 	"github.com/apex/log"
 	acli "github.com/apex/log/handlers/cli"
@@ -12,6 +15,39 @@ import (
 
 type Level = log.Level
 
+var (
+	// mu protects the output writers
+	mu sync.RWMutex
+	// stdout is the writer for progress output
+	stdout io.Writer = os.Stdout
+	// stderr is the writer for log output
+	stderr io.Writer = os.Stderr
+)
+
+// SetOutput configures the output writers for the log package.
+// stdout is used for progress output, stderr is used for log messages.
+func SetOutput(out, err io.Writer) {
+	mu.Lock()
+	defer mu.Unlock()
+	stdout = out
+	stderr = err
+	pterm.SetDefaultOutput(out)
+}
+
+// GetStdout returns the configured stdout writer.
+func GetStdout() io.Writer {
+	mu.RLock()
+	defer mu.RUnlock()
+	return stdout
+}
+
+// GetStderr returns the configured stderr writer.
+func GetStderr() io.Writer {
+	mu.RLock()
+	defer mu.RUnlock()
+	return stderr
+}
+
 func DisableStyling() {
 	pterm.DisableColor()
 	pterm.DisableStyling()
@@ -24,10 +60,21 @@ func DisableStyling() {
 }
 
 func Init() {
-	log.SetHandler(acli.Default)
+	mu.RLock()
+	w := stderr
+	mu.RUnlock()
+	log.SetHandler(acli.New(w))
 	log.SetLevel(log.InfoLevel)
 }
 
+func Infof(format string, args ...interface{}) {
+	log.Infof(format, args...)
+}
+
+func Info(arg string) {
+	log.Info(arg)
+}
+
 func Debugf(format string, args ...interface{}) {
 	DebugReal(fmt.Sprintf(format, args...), 2)
 }
@@ -55,14 +102,13 @@ func EnableDebugLogging() {
 
 func VerbosityStepsToLogLevel(l int) log.Level {
 	switch l {
-	case 1:
-		return log.WarnLevel
-	case 2:
+	case 0:
 		return log.InfoLevel
-	case 3:
+	case 1:
 		return log.DebugLevel
 	}
-	return log.ErrorLevel
+	// -vv or more
+	return log.DebugLevel
 }
 
 func SetLevelFromVerbosity(l int) {
@@ -87,3 +133,14 @@ func GetLevel() log.Level {
 func WithError(e error) *log.Entry {
 	return GetLogger().WithError(e)
 }
+
+// Progressf prints a progress message that overwrites the current line.
+// Use ProgressDone() when progress is complete to move to the next line.
+func Progressf(format string, args ...interface{}) {
+	pterm.Printf("\r"+format, args...)
+}
+
+// ProgressDone completes a progress line by printing a newline.
+func ProgressDone() {
+	pterm.Println()
+}