Add bounded concurrent scanner

Single-writer controller with a select loop that offers jobs and accepts
results simultaneously, so bounded channels cannot deadlock. statx/fstatat
relative to the directory fd on Linux/macOS, portable fallback elsewhere.
Hard links counted once, symlink loops detected, virtual filesystems
skipped, errors classified and surfaced.

internal/platform/readdir_other.go

@@ -0,0 +1,31 @@
+//go:build !linux && !darwin && !freebsd
+
+package platform
+
+import "os"
+
+// ReadDir streams the entries of a directory using the portable os API.
+// Allocated size, inode identity and link counts are not available here.
+func ReadDir(path string, chunk int, fn ChunkFunc) error {
+	return readDirGeneric(path, chunk, fn, func(p string) (Meta, error) { return statPath(p, false) })
+}
+
+func statPath(path string, follow bool) (Meta, error) {
+	var fi os.FileInfo
+	var err error
+	if follow {
+		fi, err = os.Stat(path)
+	} else {
+		fi, err = os.Lstat(path)
+	}
+	if err != nil {
+		return Meta{}, err
+	}
+	m := fromFileInfo(fi)
+	m.Name = baseName(path)
+	fillSys(&m, fi)
+	return m, nil
+}
+
+// IsVirtualFS is Linux-specific; elsewhere nothing is skipped.
+func IsVirtualFS(string) bool { return false }