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.

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
-executable, into the git-ignored `build/`). Use the targets rather than the
+with coverage and a timeout), `make check` (all three), `make build` (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