v0.15.1: EOF/Escape fixes, box title rendering, full feature verification

Bug fixes:
  - read-raw-byte now returns (values nil :eof) on stdin EOF
    instead of just nil, so callers can distinguish EOF from
    timeout.  Previously, non-TTY stdin (pipes, /dev/null)
    caused a busy-spin: sb-posix:read returned 0 immediately,
    read-raw-byte returned nil, the demo loop treated nil as
    'no event yet' and spun at 100% CPU producing 86MB of
    repeated rendering frames.

  - %read-escape-sequence now uses a 50ms timeout on the first
    follow-up byte to resolve the classic Escape-key ambiguity:
    a lone Escape press returned an :escape key-event instead of
    blocking indefinitely on VMIN=1 VTIME=0.  All callers
    (SS3, CSI, Alt+char) propagate :eof instead of faking
    :escape events when EOF occurs mid-sequence.

  - parse-csi-params now uses multiple-value-bind on read-raw-byte
    to preserve the :eof signal through CSI parsing.

  - simple-backend draw-border now renders :title on the top
    edge instead of declaring it (ignore).  The title was
    silently swallowed — the box rendered with the right border
    frame but the title text was never written.

  - demo.lisp: removed 'q' as quit key (conflicted with text
    input).  Only Esc and Ctrl+C quit.  Widget event forwarding
    scoped to tab 1 (Widgets tab).  EOF handling in main loop.
  - Stale help text (still said 'q/esc: quit') updated.

Verification infrastructure:
  - PTY-based demo test (17 checks) spawns the demo in a real
    pseudo-terminal, sends actual keystrokes, reads terminal
    output back.  Verifies: startup rendering, tab switching,
    key dispatch, 'q' doesn't quit, Escape quits via timeout,
    Ctrl+C quits, EOF clean exit, no busy-spin.

  - API feature verification (29 checks) exercises every major
    component through the actual exported API: Simple backend,
    Box with title, Text attributes, draw-rect, TextInput
    (insert/backspace/cursor/Ctrl-A/E), TextArea, key/mouse
    events, Layout flex, Markdown, Theme presets (dark/light/
    nord), Select filtering, Dialog stack, Mouse hit-test,
    Framebuffer, Dirty tracking, Modern backend, draw-ellipsis/
    draw-link, Render dispatch, Detection, Capabilities.

  - Testing pattern saved as skill (tui-pty-testing) for reuse.

Unit tests: 392/392 passing.  All 12 test suites green.

README.org

@@ -59,7 +59,7 @@ class. Programs never call terminal codes directly:
 (draw-link backend x y string url &key fg bg)
 
 ;; Input
-(read-event backend &key timeout)       → key-event or mouse-event
+(read-event backend &key timeout)       → key-event, mouse-event, :eof, or nil
 (backend-size backend)                   → (values columns lines)
 
 ;; Cursor
@@ -86,7 +86,8 @@ class. Programs never call terminal codes directly:
                    (setf running nil)))
                (mouse-event
                  ;; handle mouse
-                 ))))
+                 ))
+             (when (eq event :eof) (setf running nil))))
   (shutdown-backend be))
 ```