Add atomic writes, humanized sizes, debug logging, and -v/-q per-command

- Atomic writes for mfer gen: writes to temp file, renames on success,
  cleans up temp on error/interrupt. Prevents empty manifests on Ctrl-C.
- Humanized byte sizes using dustin/go-humanize (e.g., "10 MiB" not "10485760")
- Progress lines clear when done (using ANSI escape \r\033[K])
- Debug logging when files are added to manifest (mfer gen -vv)
- Move -v/-q flags from global to per-command for better UX
- Add tests for atomic write behavior with failing filesystem mock

README.md

@@ -356,6 +356,7 @@ The manifest file would do several important things:
 
 ## Medium Priority
 
+- [x] **Atomic writes for `mfer gen`** - Writes to temp file then atomic rename; cleans up temp file on error/interrupt.
 - [ ] **Change FileProgress callback to channel** - `mfer/builder.go` uses a callback for progress reporting; should use channels like `EnumerateStatus` and `ScanStatus` for consistency.
 - [ ] **Consolidate legacy manifest code** - `mfer/manifest.go` has old scanning code (`Scan()`, `addFile()`) that duplicates the new `internal/scanner` + `mfer/builder.go` pattern.
 - [ ] **Add context cancellation to legacy code** - The old `manifest.Scan()` doesn't support context cancellation; the new scanner does.