Add Bubble Tea TUI with interactive treemap

Cell canvas renderer with frame cache and a perceptual 256-colour
quantizer; responsive geometry (large/medium/small/too-small);
squarified treemap with nested previews, LOD grouping, category
colouring and adaptive labels; spatial arrow navigation with stable
NodeID selection; zoom and breadcrumbs; modal system (info, help,
confirm, save); list, file-type, top-N, scan-info, duplicate and
comparison views; live search/filter; mouse support; move-to-trash.

internal/platform/ops.go

@@ -0,0 +1,32 @@
+package platform
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// startDetached runs a helper program with an argument vector (never a
+// shell), detached from the terminal so it cannot disturb the TUI.
+func startDetached(name string, args ...string) error {
+	cmd := exec.Command(name, args...)
+	cmd.Stdin, cmd.Stdout, cmd.Stderr = nil, nil, nil
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	go cmd.Wait()
+	return nil
+}
+
+// checkAbs guards helper invocations: an absolute path can never be parsed
+// as an option by the helper program.
+func checkAbs(path string) error {
+	if !filepath.IsAbs(path) {
+		return errors.New("refusing relative path")
+	}
+	if _, err := os.Lstat(path); err != nil {
+		return err
+	}
+	return nil
+}