build: add make coverage target via undercover

I want to track test coverage as I work through the upstream issue backlog, so I added an undercover-based flow that mirrors how `make test-unit` already runs each file in its own Cask Emacs process.

The Makefile gets `make coverage` and `make coverage-clean`. A new helper at `tests/run-coverage-file.el` instruments `org-drill.el` before the source is loaded. Undercover merges per-file results into a single simplecov JSON at `.coverage/simplecov.json`. I added `undercover` as a Cask development dep and `.coverage/` to `.gitignore` so the report stays local.

I also renamed `make install` to `make setup`. The old name read like "deploy the package onto my system," but the target only installs Cask deps into the local `.cask/` directory. `setup` is closer to what it actually does, and all the internal `: install` prerequisites move with it.

Baseline at this commit is 10.8% (208/1928 lines on org-drill.el).

1 files changed, 33 insertions, 0 deletions

diff --git a/tests/run-coverage-file.el b/tests/run-coverage-file.el
new file mode 100644
index 0000000..e328a3f
--- /dev/null
+++ b/tests/run-coverage-file.el
@@ -0,0 +1,33 @@
+;;; run-coverage-file.el --- Undercover setup for per-file coverage runs -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Loaded by `make coverage' before each test file runs, BEFORE
+;; org-drill.el is loaded.  Instrumenting must happen first so the
+;; subsequent `(require 'org-drill)' or `-l org-drill.el' picks up the
+;; instrumented source.
+;;
+;; Coverage data is merged across per-file invocations into a single
+;; simplecov JSON at .coverage/simplecov.json.
+
+;;; Code:
+
+(unless (require 'undercover nil t)
+  (message "")
+  (message "ERROR: undercover not installed.")
+  (message "Add `undercover' to Cask as a development dep, then run `make setup'.")
+  (message "")
+  (kill-emacs 1))
+
+;; Force coverage collection in non-CI environments.  Must be set after
+;; loading undercover because the library's top-level form
+;; `(setq undercover-force-coverage (getenv "UNDERCOVER_FORCE"))' would
+;; otherwise overwrite the value.
+(setq undercover-force-coverage t)
+
+(undercover "org-drill.el"
+            (:report-format 'simplecov)
+            (:report-file ".coverage/simplecov.json")
+            (:merge-report t)
+            (:send-report nil))
+
+;;; run-coverage-file.el ends here