Mark superseded commits honestly instead of skipped (closes #152)

Gitea cancels an in-flight run when a newer commit lands on the same
branch and records the cancellation as `failure` / "Has been
cancelled". The workflow rewrote that to `skipped`, but Gitea's
Combine() folds `skipped` into `success`, so the combined-status API
returned green for a commit nothing had ever tested. Rewrite it to
`failure` / "Superseded by a newer commit; never tested" instead:
red-but-honest, and never `pending`, which would block the commit
forever.

Re-running the superseded commit would have been better still, but is
not reachable on this Gitea (1.25.4): its API exposes no rerun
endpoint, workflow dispatch takes a ref rather than a SHA, and every
replay would be a full uncached build with no bound on how many pile
up behind a burst of merges.

The step also stops hardcoding its status context: the logic moves into
script/ci-mark-superseded, which derives the context from the workflow
name, job name and event -- the same three values Gitea builds it from
-- and fails loudly when no status on the commit being built carries
that context, so renaming the workflow or the job cannot silently
disable the rewrite. That is item 2 of
#147; item 1 there is
untouched.

Tests drive the script against a fake Gitea covering the cancelled,
laundered-skipped, genuinely-failed, passing and renamed cases, so jq
joins the builder image to run them.

internal/ciscript/fakegitea_test.go

@@ -0,0 +1,142 @@
+package ciscript_test
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+)
+
+// commitStatus is the part of an entry in Gitea's combined-status
+// response that script/ci-mark-superseded reads.
+type commitStatus struct {
+	Context     string `json:"context"`
+	Status      string `json:"status"`
+	Description string `json:"description"`
+}
+
+// postedStatus is the part of a create-status request body the script
+// writes.
+type postedStatus struct {
+	Context     string `json:"context"`
+	State       string `json:"state"`
+	Description string `json:"description"`
+}
+
+// fakeGitea serves the two endpoints the script talks to. Like Gitea,
+// the newest status for a context replaces the previous one, so a
+// second run of the script sees what the first one wrote.
+type fakeGitea struct {
+	mu       sync.Mutex
+	statuses map[string][]commitStatus
+	posted   map[string][]postedStatus
+}
+
+// newFakeGitea returns the fake and the base URL to hand the script as
+// GITHUB_API_URL.
+func newFakeGitea(t *testing.T) (*fakeGitea, string) {
+	t.Helper()
+
+	fake := &fakeGitea{
+		mu:       sync.Mutex{},
+		statuses: map[string][]commitStatus{},
+		posted:   map[string][]postedStatus{},
+	}
+
+	srv := httptest.NewServer(fake.routes())
+	t.Cleanup(srv.Close)
+
+	return fake, srv.URL
+}
+
+func (f *fakeGitea) routes() http.Handler {
+	mux := http.NewServeMux()
+
+	mux.HandleFunc(
+		"GET /repos/{owner}/{repo}/commits/{sha}/status",
+		f.handleCombined,
+	)
+	mux.HandleFunc(
+		"POST /repos/{owner}/{repo}/statuses/{sha}",
+		f.handleCreate,
+	)
+
+	return mux
+}
+
+func (f *fakeGitea) handleCombined(
+	w http.ResponseWriter, r *http.Request,
+) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	body := struct {
+		Statuses []commitStatus `json:"statuses"`
+	}{Statuses: f.statuses[r.PathValue("sha")]}
+
+	payload, err := json.Marshal(body)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+
+	_, _ = w.Write(payload)
+}
+
+func (f *fakeGitea) handleCreate(w http.ResponseWriter, r *http.Request) {
+	var got postedStatus
+
+	err := json.NewDecoder(r.Body).Decode(&got)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+
+		return
+	}
+
+	sha := r.PathValue("sha")
+
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	f.posted[sha] = append(f.posted[sha], got)
+	f.replaceLocked(sha, commitStatus{
+		Context:     got.Context,
+		Status:      got.State,
+		Description: got.Description,
+	})
+
+	w.WriteHeader(http.StatusCreated)
+}
+
+// setStatus gives a commit its latest status for a context.
+func (f *fakeGitea) setStatus(sha string, status commitStatus) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	f.replaceLocked(sha, status)
+}
+
+// postedFor returns the statuses the script created for a commit.
+func (f *fakeGitea) postedFor(sha string) []postedStatus {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+
+	return append([]postedStatus(nil), f.posted[sha]...)
+}
+
+// replaceLocked requires f.mu.
+func (f *fakeGitea) replaceLocked(sha string, status commitStatus) {
+	for i, existing := range f.statuses[sha] {
+		if existing.Context == status.Context {
+			f.statuses[sha][i] = status
+
+			return
+		}
+	}
+
+	f.statuses[sha] = append(f.statuses[sha], status)
+}