build: adopt the mandated test target pattern (closes #2)

The test: target was a bare `go test $(GO_PKGS)`, diverging from the
mandated shape in four ways: no -timeout 30s, no -race, no -cover, and no
conditional verbose rerun. It now runs

    go test -timeout 30s -race -cover $(GO_PKGS)

and, on failure, reruns with -v and then exits 1 — so the build still
fails even if a flaky test happens to pass on the second attempt. The
repo's existing $(GO_PKGS) variable is kept rather than hardcoding ./...,
and the recipe is @-prefixed so the rerun banner is the only noise.

The substance here is -race, not the Makefile edit: this is the first
time the suite has run under the race detector. It is clean, across five
consecutive uncached runs, including the tcell terminal layer and the
os.Exit-path playthrough tests that were the suspected risk.

Timing against the 20-second budget: 5.1s cold (including the race
build), ~2.3s warm. The failure path was exercised with a throwaway
failing test to confirm the verbose rerun fires and make exits non-zero.

Build tooling only; no game behavior change. .golangci.yml is untouched.

Makefile

@@ -30,6 +30,10 @@ fmt-check:
 lint:
 	golangci-lint run $(GO_PKGS)
 
-# Run the test suite.
+# Run the test suite. Quiet on success; on failure, rerun verbosely for the
+# full output and still fail the target (the first run already proved the
+# tests are broken, so a flaky pass on the rerun must not rescue the build).
 test:
-	go test $(GO_PKGS)
+	@go test -timeout 30s -race -cover $(GO_PKGS) || \
+		{ echo "--- Rerunning with -v for details ---"; \
+		  go test -timeout 30s -race -v $(GO_PKGS); exit 1; }