build: add a make cover target for per-function coverage (closes #17)

make cover writes build/coverage.out and prints the per-function report;
make cover-html renders the same profile to build/coverage.html. The
per-package percentage make test prints cannot say which function is
untested.

Both write files, so neither is in check, and neither may be added to it:
check must not modify the working tree. Their output lands under the
already-ignored build/.

Verified: make cover printed per-function lines and a 62.6% total,
make cover-html wrote build/coverage.html, and git status stayed clean.
GOFLAGS=-count=1 make check green in 24s with the lint layer executing
(12.0s, "0 issues.", not CACHED) and the suite running for real (cmd/rogue
1.037s, game 3.410s); check is still fmt-check lint test and git status is
clean afterwards.

Makefile

@@ -14,8 +14,10 @@ PRETTIER  := prettier --tab-width 4 --prose-wrap always
 # git-ignored. Targets that write outside it can commit their output.
 BUILD_DIR := build
 BIN       := $(BUILD_DIR)/rogue
+COVERPROF := $(BUILD_DIR)/coverage.out
+COVERHTML := $(BUILD_DIR)/coverage.html
 
-.PHONY: build check fmt fmt-check lint test
+.PHONY: build check cover cover-html fmt fmt-check lint test
 
 # Format, lint, and test — the full local pre-commit gate. Keep this list
 # to targets that write nothing into the working tree.
@@ -27,6 +29,19 @@ build:
 	@mkdir -p $(BUILD_DIR)
 	go build -o $(BIN) ./cmd/rogue
 
+# Per-function coverage, for finding which functions are untested. The
+# percentage `make test` prints is a per-package total and cannot answer
+# that. Writes files, so it stays out of `check`.
+cover:
+	@mkdir -p $(BUILD_DIR)
+	go test -timeout 30s -coverprofile=$(COVERPROF) $(GO_PKGS)
+	go tool cover -func=$(COVERPROF)
+
+# Render the same profile as annotated source.
+cover-html: cover
+	go tool cover -html=$(COVERPROF) -o $(COVERHTML)
+	@echo "wrote $(COVERHTML)"
+
 # Format Go and Markdown in place.
 fmt:
 	gofmt -w .