feat(playground): add Ebox layout examples

Provide a standalone public-API-only Ebox playground demonstrating Grid layout and buffer lifecycle with tests and bilingual README documentation.

.gitignore

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+*.elc
+tests/*.elc

Makefile

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+EMACS ?= emacs
+LOAD_PATH = -L . -L ../emacs-box
+
+.PHONY: all compile test check checkdoc load clean
+
+all: check
+
+compile:
+	rm -f *.elc tests/*.elc
+	$(EMACS) -Q --batch $(LOAD_PATH) --eval '(setq load-prefer-newer t)' \
+		--eval '(load-file "ebox-playground.el")' \
+		--eval '(byte-compile-file "ebox-playground.el")'
+
+test: compile
+	$(EMACS) -Q --batch $(LOAD_PATH) --eval '(setq load-prefer-newer t)' \
+		-l tests/ebox-playground-tests.el -f ert-run-tests-batch-and-exit
+
+load: compile
+	$(EMACS) -Q --batch $(LOAD_PATH) --eval '(require (quote ebox-playground))' \
+		--eval '(princ "ebox-playground load OK\n")'
+
+checkdoc:
+	$(EMACS) -Q --batch --eval '(progn (require (quote checkdoc)) (dolist (file (directory-files "." t)) (when (string-suffix-p ".el" file) (checkdoc-file file))))'
+
+check: checkdoc compile test
+
+clean:
+	rm -f *.elc tests/*.elc

README.md

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+# ebox-playground
+
+`ebox-playground` is the independent developer example package for Ebox. It uses only public Ebox constructors and demonstrates fixed, fractional, two-dimensional, and placed layout. It is separate from `etaf-playground`, which demonstrates the higher-level Component and Runtime framework.
+
+```elisp
+(require 'ebox-playground)
+(ebox-playground-open)
+```
+
+Run `make check EMACS=/Applications/Emacs.app/Contents/MacOS/Emacs` from this directory.

README.zh-CN.md

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+# ebox-playground
+
+`ebox-playground` 是 Ebox 的独立开发示例包，只使用 Ebox 公共构造函数，展示固定轨道、分数轨道、二维 Grid 和显式放置。它与展示高层 Component/Runtime 框架的 `etaf-playground` 分开。
+
+```elisp
+(require 'ebox-playground)
+(ebox-playground-open)
+```
+
+在该目录运行 `make check EMACS=/Applications/Emacs.app/Contents/MacOS/Emacs`。

ebox-playground.el

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+;;; ebox-playground.el --- Public Ebox layout examples -*- lexical-binding: t; -*-
+
+;; SPDX-License-Identifier: GPL-3.0-or-later
+;; Author: Ebox contributors
+;; Version: 0.1.0
+;; Package-Requires: ((emacs "29.1") (ebox "1.0.1"))
+;; URL: https://github.com/ginqi7/ebox-playground
+
+;;; Commentary:
+
+;; This package is intentionally small and public-only.  It demonstrates the
+;; Ebox contract without reaching into ebox-- internals.  ETAF's application
+;; examples live in the sibling `etaf-playground' package.
+
+;;; Code:
+
+(require 'ebox)
+
+(defconst ebox-playground-buffer-name "*Ebox Playground*"
+  "Default buffer name used by `ebox-playground-open'.")
+
+;;;###autoload
+(defun ebox-playground-view ()
+  "Return a public Ebox example layout node without buffer side effects."
+  (ebox-grid
+   :width '(640)
+   :grid-template-columns '((200) 1fr)
+   :grid-template-rows '(1 1 1)
+   :gap '(1 (12))
+   :padding '(1 2)
+   :border '((1) solid "#687386")
+   (ebox-create :content "Ebox Playground" :face 'bold)
+   (ebox-create :content "A pixel-precise two-dimensional layout")
+   (ebox-create :content "Row / column / flex / grid share the public node contract")
+   (ebox-create :content "Fixed and fractional tracks")
+   (ebox-create :content "Placement" :grid-column 1 :grid-row 3)
+   (ebox-create :content "No private calls" :grid-column 2 :grid-row 3)))
+
+;;;###autoload
+(defun ebox-playground-open (&optional buffer-name)
+  "Render the public Ebox example into BUFFER-NAME and return its buffer."
+  (interactive)
+  (ebox-render-to-buffer (or buffer-name ebox-playground-buffer-name)
+                         (ebox-playground-view)))
+
+;;;###autoload
+(defun ebox-playground-close (&optional buffer-name)
+  "Kill the playground BUFFER-NAME and return its former buffer."
+  (interactive)
+  (let ((buffer (get-buffer (or buffer-name ebox-playground-buffer-name))))
+    (when buffer
+      (kill-buffer buffer))
+    buffer))
+
+(provide 'ebox-playground)
+
+;;; ebox-playground.el ends here

tests/ebox-playground-tests.el

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+;;; ebox-playground-tests.el --- Public Ebox example tests -*- lexical-binding: t; -*-
+
+(require 'ert)
+(require 'ebox-playground)
+
+(ert-deftest ebox-playground-uses-only-public-layout-apis ()
+  "The example should build and render through the public Ebox entry point."
+  (let ((node (ebox-playground-view)))
+    (should (eq (plist-get node :ebox-type) 'grid))
+    (should (string-match-p "Ebox Playground"
+                            (substring-no-properties (ebox-render node))))))
+
+(ert-deftest ebox-playground-opens-and-closes-buffer ()
+  "The public open and close commands should own their buffer lifecycle."
+  (let ((name " *ebox-playground-test*"))
+    (unwind-protect
+        (progn
+          (ebox-playground-open name)
+          (should (buffer-live-p (get-buffer name)))
+          (with-current-buffer name
+            (should (string-match-p "Ebox Playground" (buffer-string)))))
+      (ebox-playground-close name))))
+
+(provide 'ebox-playground-tests)
+
+;;; ebox-playground-tests.el ends here