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).

4 files changed, 95 insertions, 9 deletions

diff --git a/.gitignore b/.gitignore
index 7f89e14..f7fabe8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,5 @@ org-drill.html
 /docs
 
 todo.org
+/.ai/
+/.coverage/
diff --git a/Cask b/Cask
index 5eaf201..c9ebcb2 100644
--- a/Cask
+++ b/Cask
@@ -6,6 +6,7 @@
 (package-file "org-drill.el")
 
 (development
- (depends-on "assess"))
+ (depends-on "assess")
+ (depends-on "undercover"))
 (depends-on "org" "9.2")
 
diff --git a/Makefile b/Makefile
index b4ece5d..9f506b4 100644
--- a/Makefile
+++ b/Makefile
@@ -9,7 +9,7 @@
 #   make robot             - Run basic robot tests
 #   make robot-all         - Run all robot tests
 #   make docker-test       - Run tests in Docker (multiple Emacs versions)
-#   make install           - Install dependencies via Cask
+#   make setup             - Install dependencies via Cask
 #   make clean             - Remove generated files
 
 # Emacs binary to use (override with: make EMACS=emacs29 test)
@@ -38,9 +38,14 @@ EMACS_BATCH = $(EMACS) --batch --no-site-file --no-site-lisp
 # Docker configuration
 DOCKER_TAG=26
 
-.PHONY: help test test-all test-unit test-integration test-file test-name install build clean clean-elc
+# Coverage configuration
+COVERAGE_DIR = .coverage
+COVERAGE_FILE = $(COVERAGE_DIR)/simplecov.json
+
+.PHONY: help test test-all test-unit test-integration test-file test-name setup build clean clean-elc
 .PHONY: robot robot-all robot-basic robot-leitner robot-all-card robot-spanish robot-explainer
 .PHONY: docker-test test-cp test-git
+.PHONY: coverage coverage-clean
 
 # Default target
 help:
@@ -58,8 +63,12 @@ help:
 	@echo "  make test-file FILE=<filename>  - Run specific test file"
 	@echo "  make test-name TEST=<pattern>   - Run tests matching pattern"
 	@echo ""
+	@echo "Coverage:"
+	@echo "  make coverage          - Generate simplecov JSON at $(COVERAGE_FILE)"
+	@echo "  make coverage-clean    - Delete the coverage report file"
+	@echo ""
 	@echo "Advanced Targets:"
-	@echo "  make install           - Install dependencies via Cask"
+	@echo "  make setup             - Install dependencies via Cask"
 	@echo "  make build             - Build package via Cask"
 	@echo "  make docker-test       - Run tests in Docker (multiple Emacs versions)"
 	@echo "  make clean             - Remove generated files"
@@ -82,7 +91,7 @@ help:
 all: robot test-unit
 
 # Install dependencies
-install:
+setup:
 	@if ! command -v $(CASK) >/dev/null 2>&1; then \
 		echo "[✗] Cask not found. Please install Cask first:"; \
 		echo "    https://github.com/cask/cask"; \
@@ -104,7 +113,7 @@ test: robot test-unit
 test-all: robot-all test-unit test-integration
 
 # Run unit tests only
-test-unit: install
+test-unit: setup
 	@echo "[i] Running unit tests ($(words $(UNIT_TESTS)) files)..."
 	@failed=0; \
 	for test in $(UNIT_TESTS); do \
@@ -124,7 +133,7 @@ test-unit: install
 	fi
 
 # Run integration tests only
-test-integration: install
+test-integration: setup
 	@echo "[i] Running integration tests ($(words $(INTEGRATION_TESTS)) files)..."
 	@if [ $(words $(INTEGRATION_TESTS)) -eq 0 ]; then \
 		echo "[i] No integration tests found"; \
@@ -149,7 +158,7 @@ test-integration: install
 
 # Run specific test file
 # Usage: make test-file FILE=org-drill-test.el
-test-file: install
+test-file: setup
 ifndef FILE
 	@echo "[✗] Error: FILE parameter required"
 	@echo "Usage: make test-file FILE=org-drill-test.el"
@@ -167,7 +176,7 @@ endif
 # Run specific test by name/pattern
 # Usage: make test-name TEST=load-test
 #        make test-name TEST="find-*"
-test-name: install
+test-name: setup
 ifndef TEST
 	@echo "[✗] Error: TEST parameter required"
 	@echo "Usage: make test-name TEST=load-test"
@@ -184,6 +193,47 @@ endif
 	@echo "[✓] Tests matching '$(TEST)' complete"
 
 #
+# Coverage (undercover + simplecov JSON)
+#
+# Each unit-test file runs in its own Emacs process (matching `make
+# test-unit'); run-coverage-file.el instruments org-drill.el before the
+# source is loaded, and undercover merges per-file results into a single
+# simplecov JSON.
+
+coverage: coverage-clean setup $(COVERAGE_DIR)
+	@echo "[i] Cleaning .elc files so undercover can instrument source..."
+	@find . -name "*.elc" -delete
+	@echo "[i] Running coverage across $(words $(UNIT_TESTS)) unit-test file(s)..."
+	@echo "    (slower than 'make test-unit' — each file runs in its own Emacs)"
+	@failed=0; \
+	for test in $(UNIT_TESTS); do \
+		echo "  Coverage: $$test..."; \
+		$(EMACS_ENV) $(CASK) emacs --batch -q \
+			-l ert \
+			-l assess \
+			-l $(TEST_DIR)/run-coverage-file.el \
+			-l org-drill.el \
+			-l $$test \
+			-f ert-run-tests-batch-and-exit || failed=$$((failed + 1)); \
+	done; \
+	if [ $$failed -gt 0 ]; then \
+		echo "[!] $$failed test file(s) failed during coverage run"; \
+		exit 1; \
+	fi
+	@if [ -f $(COVERAGE_FILE) ]; then \
+		echo "[✓] Coverage report: $(COVERAGE_FILE) ($$(du -h $(COVERAGE_FILE) | cut -f1))"; \
+	else \
+		echo "[!] No coverage file produced; check that undercover is installed"; \
+		exit 1; \
+	fi
+
+coverage-clean:
+	@rm -f $(COVERAGE_FILE)
+
+$(COVERAGE_DIR):
+	@mkdir -p $(COVERAGE_DIR)
+
+#
 # Robot Tests (Automated UI Tests)
 #
 
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