Rename cl-tui -> cl-tty, v0.9.0: Dialog System + Toast

Rename: cl-tty avoids naming collision with Quicklisp's cl-tui (naryl/cl-tui,
a cl-charms-based ncurses library). Our project is pure escape-sequence CL.

v0.9.0 adds:
- Dialog base class: modal overlay with backdrop, centered panel, size
  variants (:small/:medium/:large), stack-based management
- Dialog subclasses: alert, confirm, select-dialog, prompt-dialog
- Toast notifications: transient, top-right corner, auto-dismiss,
  colored variants (info/success/warning/error)
- 78 tests total, 100% passing

ASDF: read-time package references (+fiveam:+) replaced with
find-symbol so .asd loads without FiveAM pre-loaded

backend/modern.lisp

@@ -8,7 +8,7 @@
 ;;   sgr-fg sgr-bg sgr-attr cursor-move-escape cursor-style-escape
 ;;   decicm-begin decicm-end osc8-link hex-to-rgb border-char
 
-(in-package :cl-tui.backend)
+(in-package :cl-tty.backend)
 
 (defun hex-to-rgb (hex)
   "Parse a hex color string like \"#FFD700\" into (values r g b).