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

contrib/usage.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+set -euo pipefail
+
+# usage.sh - Generate and check a manifest from the repo
+# Run from repo root: ./contrib/usage.sh
+
+TMPDIR=$(mktemp -d)
+MANIFEST="$TMPDIR/index.mf"
+
+cleanup() {
+    rm -rf "$TMPDIR"
+}
+trap cleanup EXIT
+
+echo "Building mfer..."
+go build -o "$TMPDIR/mfer" ./cmd/mfer
+
+"$TMPDIR/mfer" generate --ignore-dotfiles -o "$MANIFEST" .
+"$TMPDIR/mfer" check --base . "$MANIFEST"