Critical fixes: case→cond in %read-event, theme resolution, SGR mouse, scrollbox/text-input/textarea render stubs, test runner exit code, ASDF rename

CRITICAL: case b → cond in %read-event (input.lisp:280)
  case with (and ...) predicate clauses treats keys as eql-compared
  atoms — all range clauses were dead code.  Every Ctrl+letter and
  printable ASCII fell through to :unknown.  text-input/textarea
  widgets were non-functional with real terminal input.  No test
  coverage of %read-event masked this.

HIGH: Theme resolution wired (backend/modern.lisp, theme.lisp)
  sgr-fg/sgr-bg now fall back to *theme-colors* hash for semantic
  keywords (:accent, :text-muted, :background-element).  *theme-colors*
  exported from cl-tty.backend.  load-preset populates it from preset
  hex values.  Previously all themed render output was invisible.

HIGH: SGR mouse parser wired (input.lisp:210-215)
  parse-sgr-mouse was defined but never called.  Now %read-escape-sequence
  detects ESC[< prefix and routes to parse-sgr-mouse.  Mouse drags,
  releases, and scroll events now parse correctly.

MEDIUM: Rendering stubs replaced
  - scrollbox: delegates to (render child backend) with position
    offset via unwind-protect (was debug string 'child at ~D')
  - text-input: draws value/placeholder at layout position
  - textarea: draws visible lines at layout position

MEDIUM: hit-test uses component-layout-node (mouse.lisp:18-31)
  Was checking nonexistent x/y/width/height slots.  Now reads
  layout-node-x/y/w/h via component-layout-node generic.

MEDIUM: test runner exit code (run-all-tests.lisp, cl-tty.asd)
  run-all-tests.lisp exits 1 if any suite fails.
  asdf:test-system exits 1 on failure.
  Renamed :cl-tty-tests to :cl-tty/test (ASDF convention).

MEDIUM: draw-border respects x/y on simple-backend (simple.lisp:42-53)
  Was writing to cursor position only.  Now uses newlines+spaces
  to reach specified coordinates (no escape sequences needed).

LOW: TabBar truncation off-by-one fixed (tabbar.lisp:47)
  >= changed to > to avoid cutting tabs 2 chars early.

LOW: Scrollbar coordinates absolute (scrollbox.lisp:61-73)
  Scrollbar drawn at viewport-relative (0,0).  Now adds layout
  node x/y offset for correct terminal positioning.

LOW: backend-write calls finish-output (modern.lisp:169)

LOW: load-preset no longer flips theme-mode (theme.lisp:43-45)
  Mode toggle caused load-preset to load wrong variant on
  second call.

All backported to org source files (org/text-input.org,
org/scrollbox-tabbar.org) so tangling produces matching .lisp.

392 tests pass, exit code 0.

src/components/scrollbox.lisp

@@ -39,6 +39,8 @@
           :initial-value 0))
 
 (defmethod render ((sb scroll-box) backend)
+  "Render ScrollBox children within the viewport, offset by scroll position.
+Children outside the viewport are skipped."
   (let* ((ln (scroll-box-layout-node sb))
          (vx 0) (vy 0)
          (vw (if ln (layout-node-width ln) 80))
@@ -49,9 +51,20 @@
       (let* ((cln (component-layout-node child))
              (ch (if cln (layout-node-height cln) 1))
              (cy vy))
-        (when (and (< (+ cy (- sy)) (+ vh vy)) (> (+ cy (- sy) ch) vy))
-          (draw-text backend (- sx) (+ vy cy (- sy))
-                     (format nil "child at ~D" vy) nil nil))
+        ;; Only render children that are visible in the viewport
+        (when (and (< (+ cy (- sy)) (+ vh vy))
+                   (> (+ cy (- sy) ch) vy))
+          ;; Temporarily offset child's layout-node position for rendering
+          (let ((orig-x (if cln (layout-node-x cln) 0))
+                (orig-y (if cln (layout-node-y cln) 0)))
+            (when cln
+              (setf (layout-node-x cln) (- orig-x sx)
+                    (layout-node-y cln) (- orig-y sy)))
+            (unwind-protect
+                 (render child backend)
+              (when cln
+                (setf (layout-node-x cln) orig-x
+                      (layout-node-y cln) orig-y)))))
         (incf vy ch)))
     (draw-scrollbars sb backend vw vh)))
 
@@ -59,18 +72,21 @@
   (if (> content-size viewport-size) (/ (float scroll-pos) (- content-size viewport-size)) 0.0))
 
 (defun draw-scrollbars (sb backend viewport-w viewport-h)
-  (let* ((content-h (scroll-box-content-height sb)) (content-w (scroll-box-content-width sb))
+  (let* ((ln (scroll-box-layout-node sb))
+         (vx (if ln (layout-node-x ln) 0))
+         (vy (if ln (layout-node-y ln) 0))
+         (content-h (scroll-box-content-height sb)) (content-w (scroll-box-content-width sb))
          (sy (scroll-box-scroll-y sb)) (sx (scroll-box-scroll-x sb)))
     (when (> content-h viewport-h)
       (let* ((thumb (scrollbar-thumb sy viewport-h content-h))
              (thumb-pos (round (* thumb viewport-h))))
-        (draw-rect backend (1- viewport-w) 0 1 viewport-h :bg :bright-black)
-        (draw-text backend (1- viewport-w) thumb-pos "█" nil nil)))
+        (draw-rect backend (+ vx (1- viewport-w)) vy 1 viewport-h :bg :bright-black)
+        (draw-text backend (+ vx (1- viewport-w)) (+ vy thumb-pos) "█" nil nil)))
     (when (> content-w viewport-w)
       (let* ((thumb (scrollbar-thumb sx viewport-w content-w))
              (thumb-pos (round (* thumb viewport-w))))
-        (draw-rect backend 0 (1- viewport-h) viewport-w 1 :bg :bright-black)
-        (draw-text backend thumb-pos (1- viewport-h) "█" nil nil)))))
+        (draw-rect backend vx (+ vy (1- viewport-h)) viewport-w 1 :bg :bright-black)
+        (draw-text backend (+ vx thumb-pos) (+ vy (1- viewport-h)) "█" nil nil)))))
 
 (defun update-sticky-scroll (sb)
   (when (sticky-scroll-p sb)