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/package.lisp

@@ -1,4 +1,4 @@
-(defpackage :cl-tui.backend
+(defpackage :cl-tty.backend
   (:use :cl)
   (:export
    ;; Backend classes
@@ -26,4 +26,4 @@
    #: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)