feat: add Go TUI rendering and terminal helper functions

7 funciones Go del dominio tui: apply_gradient (gradiente de color en texto),
draw_box y draw_separator (renderizado de cajas y separadores con box_chars),
load_ascii_art (carga de arte ASCII desde archivos), normalize_terminal_output
y strip_ansi (limpieza de salida de terminal), read_dir_autocomplete
(autocompletado de rutas de directorio). Incluye box_chars.go como helper
de caracteres Unicode para bordes.

functions/tui/read_dir_autocomplete.go

@@ -0,0 +1,32 @@
+package tui
+
+import (
+	"os"
+	"sort"
+	"strings"
+)
+
+// AutocompleteCandidate represents a directory entry for path autocomplete.
+type AutocompleteCandidate struct {
+	Name  string
+	IsDir bool
+}
+
+// ReadDirAutocomplete reads entries from searchDir whose name starts with prefix
+// (case-insensitive). Returns candidates sorted by name.
+func ReadDirAutocomplete(searchDir, prefix string) ([]AutocompleteCandidate, error) {
+	entries, err := os.ReadDir(searchDir)
+	if err != nil {
+		return nil, err
+	}
+	prefixLower := strings.ToLower(prefix)
+	var results []AutocompleteCandidate
+	for _, e := range entries {
+		name := e.Name()
+		if prefix == "" || strings.HasPrefix(strings.ToLower(name), prefixLower) {
+			results = append(results, AutocompleteCandidate{Name: name, IsDir: e.IsDir()})
+		}
+	}
+	sort.Slice(results, func(i, j int) bool { return results[i].Name < results[j].Name })
+	return results, nil
+}