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/sys_windows.go

@@ -0,0 +1,12 @@
+package platform
+
+import (
+	"os"
+	"syscall"
+)
+
+func fillSys(m *Meta, fi os.FileInfo) {
+	if d, ok := fi.Sys().(*syscall.Win32FileAttributeData); ok {
+		m.BTime = d.CreationTime.Nanoseconds()
+	}
+}