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/cli/entry.go

@@ -1,7 +1,10 @@
 package cli
 
 import (
+	"io"
 	"os"
+
+	"github.com/spf13/afero"
 )
 
 var NO_COLOR bool
@@ -13,13 +16,50 @@ func init() {
 	}
 }
 
-func Run(Appname, Version, Gitrev string) int {
-	m := &CLIApp{}
-	m.appname = Appname
-	m.version = Version
-	m.gitrev = Gitrev
-	m.exitCode = 0
+// RunOptions contains all configuration for running the CLI application.
+type RunOptions struct {
+	Appname string
+	Version string
+	Gitrev  string
+	Args    []string
+	Stdin   io.Reader
+	Stdout  io.Writer
+	Stderr  io.Writer
+	Fs      afero.Fs
+}
 
-	m.run()
+// DefaultRunOptions returns RunOptions configured for normal CLI execution.
+func DefaultRunOptions(appname, version, gitrev string) *RunOptions {
+	return &RunOptions{
+		Appname: appname,
+		Version: version,
+		Gitrev:  gitrev,
+		Args:    os.Args,
+		Stdin:   os.Stdin,
+		Stdout:  os.Stdout,
+		Stderr:  os.Stderr,
+		Fs:      afero.NewOsFs(),
+	}
+}
+
+// Run creates and runs the CLI application with default options.
+func Run(appname, version, gitrev string) int {
+	return RunWithOptions(DefaultRunOptions(appname, version, gitrev))
+}
+
+// RunWithOptions creates and runs the CLI application with the given options.
+func RunWithOptions(opts *RunOptions) int {
+	m := &CLIApp{
+		appname:  opts.Appname,
+		version:  opts.Version,
+		gitrev:   opts.Gitrev,
+		exitCode: 0,
+		Stdin:    opts.Stdin,
+		Stdout:   opts.Stdout,
+		Stderr:   opts.Stderr,
+		Fs:       opts.Fs,
+	}
+
+	m.run(opts.Args)
 	return m.exitCode
 }