v0.3.3: left/right cursor movement in input

Adds :cursor-pos to TUI state. New functions:
- input-insert-char(ch): insert at cursor position, advance cursor
- input-delete-char(): delete char before cursor (standard backspace)

on-key handlers:
- Left arrow: decrement cursor-pos (clamped >= 0)
- Right arrow: increment cursor-pos (clamped <= buffer-len)
- Character input: input-insert-char at cursor position
- Backspace: input-delete-char at cursor position
- Enter: reset cursor-pos to 0

view-input: cursor at visual position matching cursor-pos

Test: (init-state) → (input-insert-char #\h) → (input-insert-char #\i)
→ (setf cursor-pos 1) → (input-insert-char #\X) → 'hXi' at pos 2

org/gateway-tui-view.org

@@ -115,10 +115,12 @@ Returns list of trimmed strings. Single words wider than width are split."
 (defun view-input (win)
   (let* ((text (input-string))
          (w (or (width win) 78))
-         (clip (min (length text) (1- w))))
+         (pos (or (st :cursor-pos) 0))
+         (display-start (max 0 (- pos (1- w))))
+         (visible (subseq text display-start (min (length text) (+ display-start w)))))
     (clear win)
-    (add-string win (format nil "~a " text) :y 0 :x 0 :n (1- w) :fgcolor (theme-color :input))
-    (setf (cursor-position win) (list 0 clip)))
+    (add-string win (format nil "~a " visible) :y 0 :x 0 :n (1- w) :fgcolor (theme-color :input))
+    (setf (cursor-position win) (list 0 (min (- pos display-start) (1- w)))))
   (refresh win))
 #+end_src