v0.15.0: Rewrite demo, update README, fix read-raw-byte buffer, export textarea-lines

Demo (demo.lisp):
  - Full interactive demo with 3 tabs: Home, Widgets, Console
  - Uses read-event/SGR mouse paths (exercises real terminal input)
  - Demonstrates text-input, textarea, backend drawing, tab navigation
  - Event log console shows keyboard and mouse events in real time
  - Proper terminal cleanup via shutdown-backend + unwind-protect

README.org:
  - Complete rewrite with getting-started guide, architecture overview
  - API reference for all components with signatures and examples
  - Event loop pattern, layout system, rendering pipeline docs
  - Backend features table, development guide, project structure

Bug fixes:
  - read-raw-byte (input.lisp:89-109): use sb-sys:with-pinned-objects +
    vector-sap for proper sb-posix:read buffer handling (SBCL type error
    with plain (unsigned-byte 8) arrays)
  - input-package.lisp: export textarea-lines (was missing from package)

Version bump: v0.14.0 → v0.15.0

392 tests pass.

src/components/input-package.lisp

@@ -26,6 +26,7 @@
    #:textarea-value #:textarea-cursor-row #:textarea-cursor-col
    #:textarea-on-submit #:textarea-undo-stack #:textarea-redo-stack
    #:textarea-layout-node
+   #:textarea-lines
    #:handle-textarea-input #:render-textarea
    ;; Keybindings
    #:keymap #:make-keymap #:keymap-name #:keymap-bindings #:keymap-parent

src/components/input.lisp

@@ -87,26 +87,27 @@
 ;;; Low-level byte reading
 ;;; ---------------------------------------------------------------------------
 (defun read-raw-byte (&key timeout)
-  (if timeout
-      (let ((deadline (+ (get-universal-time) timeout)))
-        (loop while (< (get-universal-time) deadline)
-              do (handler-case
-                     (let ((buf (make-array 1 :element-type '(unsigned-byte 8))))
-                       (let ((n (sb-posix:read 0 buf 1)))
-                         (when (plusp n)
-                           (return-from read-raw-byte (aref buf 0)))))
-                   (sb-posix:syscall-error ()
-                     (return-from read-raw-byte nil)))
-              (sleep 0.01))
-        nil)
-      (let ((buf (make-array 1 :element-type '(unsigned-byte 8))))
-        (multiple-value-bind (n err)
-            (ignore-errors (sb-posix:read 0 buf 1))
-          (if (and (integerp n) (plusp n))
-              (aref buf 0)
-              (progn
-                (when err (format *error-output* "read error: ~A~%" err))
-                nil))))))
+  (flet ((read-one ()
+           (let ((buf (make-array 1 :element-type '(unsigned-byte 8))))
+             ;; Use sb-sys:with-pinned-objects so sb-posix:read can access the buffer
+             (sb-sys:with-pinned-objects (buf)
+               (let ((n (sb-posix:read 0 (sb-sys:vector-sap buf) 1)))
+                 (when (plusp n)
+                   (return-from read-raw-byte (aref buf 0))))))))
+    (if timeout
+        (let ((deadline (+ (get-universal-time) timeout)))
+          (loop while (< (get-universal-time) deadline)
+                do (handler-case
+                       (read-one)
+                     (sb-posix:syscall-error ()
+                       (return-from read-raw-byte nil)))
+                   (sleep 0.01))
+          nil)
+        (handler-case
+            (read-one)
+          (sb-posix:syscall-error (e)
+            (format *error-output* "read error: ~A~%" e)
+            nil)))))
 
 ;;; ---------------------------------------------------------------------------
 ;;; CSI parameter parser