chore: add Cask + ert-runner test infrastructure

Replace the bare emacs --batch test runner with cask exec ert-runner.
The Makefile test targets now delegate. validate-parens, compile, and
lint stay on bare emacs --batch. Running cask once per file would slow
them down for no real win.

Adds:
- Cask file declaring sources and ert-runner as a dev dep
- .ert-runner config adding . and tests to load-path
- tests/test-helper.el placeholder for future test setup
- .gitignore entry for .cask/

All 88 tests pass in 0.19s via the new path.

1 files changed, 17 insertions, 0 deletions

diff --git a/tests/test-helper.el b/tests/test-helper.el
new file mode 100644
index 0000000..783fafa
--- /dev/null
+++ b/tests/test-helper.el
@@ -0,0 +1,17 @@
+;;; test-helper.el --- Test setup for ert-runner -*- lexical-binding: t -*-
+
+;; SPDX-License-Identifier: GPL-3.0-or-later
+
+;;; Commentary:
+
+;; Loaded automatically by ert-runner before test files.  Adds the
+;; tests/ directory to `load-path' so `(require \\='testutil-gloss)' and
+;; sibling testutil files resolve.
+
+;;; Code:
+
+(let ((tests-dir (file-name-directory (or load-file-name buffer-file-name))))
+  (add-to-list 'load-path tests-dir))
+
+(provide 'test-helper)
+;;; test-helper.el ends here