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

lisp/gateway-tui-model.lisp

@@ -28,7 +28,7 @@
   (setf *state*
         (list :running t :mode :chat :connected nil :stream nil
               :input-buffer nil :input-history nil :input-hpos 0
-              :messages nil :scroll-offset 0 :busy nil
+              :messages nil :scroll-offset 0 :busy nil :cursor-pos 0
               :dirty (list nil nil nil))))
 
 (defun now ()
@@ -39,6 +39,25 @@
 (defun input-string ()
   (coerce (reverse (st :input-buffer)) 'string))
 
+(defun input-insert-char (ch)
+  "Insert character at cursor position into the input buffer."
+  (let* ((buf (st :input-buffer))
+         (pos (or (st :cursor-pos) 0))
+         (s (coerce (reverse buf) 'string))
+         (new (concatenate 'string (subseq s 0 pos) (string ch) (subseq s pos))))
+    (setf (st :input-buffer) (reverse (coerce new 'list)))
+    (setf (st :cursor-pos) (1+ pos))))
+
+(defun input-delete-char ()
+  "Delete character before cursor position (standard backspace)."
+  (let* ((buf (st :input-buffer))
+         (pos (or (st :cursor-pos) 0)))
+    (when (and buf (> pos 0))
+      (let* ((s (coerce (reverse buf) 'string))
+             (new (concatenate 'string (subseq s 0 (1- pos)) (subseq s pos))))
+        (setf (st :input-buffer) (reverse (coerce new 'list)))
+        (setf (st :cursor-pos) (1- pos))))))
+
 (defun add-msg (role content)
   (push (list :role role :content content :time (now)) (st :messages))
   (setf (st :dirty) (list t t nil)))