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 .

README.md

@@ -80,8 +80,10 @@ For development, the `Makefile` wraps the toolchain: `make fmt` (gofmt +
 prettier), `make lint` (`script/lint`, which runs golangci-lint inside the
 pinned container built from `Dockerfile.lint` — it is never installed on the
 host, so docker is required), `make test` (the suite, under the race detector
-with coverage and a timeout), `make check` (all three), and `make build` (the
+with coverage and a timeout), `make check` (all three), `make build` (the
-executable, into the git-ignored `build/`). Use the targets rather than the
+executable), and `make cover` / `make cover-html` (per-function coverage, and
+the same profile as annotated source at `build/coverage.html`). Everything they
+generate lands in the git-ignored `build/`. Use the targets rather than the
 toolchain directly — they carry the flags the project relies on.
 
 ## License

TODO.md

@@ -35,6 +35,16 @@ is finished.
 
 # Completed Steps
 
+- 2026-08-10 `make cover` added (https://git.eeqj.de/sneak/rgoue/issues/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, which is how the coverage gaps closed so far had to be found — by
+  grepping test files for identifiers.
+
+    Neither target is in `check`, and neither may be added to it: both write
+    files, and `make check` must not modify the working tree.
+
 - 2026-08-10 `make build` added (https://git.eeqj.de/sneak/rgoue/issues/19). The
   executable is built to `build/rogue`; `README.md` no longer contains a raw
   `go` invocation anywhere. `build` is in neither `check` nor `test` —