ui/phase-27: battle viewer (radial scene, playback, map markers)

Engine wire change: Report.battle switched from []uuid.UUID to
[]BattleSummary{id, planet, shots} so the map can place battle
markers without N extra fetches. FBS schema + generated Go/TS
regenerated; transcoder + report controller updated; openapi
adds the BattleSummary schema with a freeze test.

Backend gateway forwards engine GET /api/v1/battle/:turn/:uuid as
/api/v1/user/games/{game_id}/battles/{turn}/{battle_id} (handler
plus engineclient.FetchBattle, contract test stub, openapi spec).

UI:
- BattleViewer (lib/battle-player/) is a logically isolated SVG
  radial scene that consumes a BattleReport prop. Planet at the
  centre, races on the outer ring at equal angular spacing, race
  clusters by (race, className) with <class>:<numLeft> labels;
  observer groups (inBattle: false) are not drawn; eliminated
  races drop out and survivors re-distribute on the next frame.
- Shot line per frame: red on destroyed, green otherwise; erased
  on the next frame. Playback controls: play/pause + step ± +
  rewind + 1x/2x/4x speed (400/200/100 ms per frame).
- Page wrapper (lib/active-view/battle.svelte) loads BattleReport
  via api/battle-fetch.ts; synthetic-gameId prefix routes to a
  fixture loader, otherwise REST through the gateway. Always-
  visible <ol> text protocol satisfies the accessibility ask.
- section-battles.svelte links every battle UUID into the viewer.
- map/battle-markers.ts: yellow X cross of 2 LinePrim through the
  corners of the planet's circumscribed square (stroke width
  clamps from 1 px at 1 shot to 5 px at 100+ shots); bombing
  marker is a stroke-only ring (yellow when damaged, red when
  wiped). Wired into state-binding.ts; click handler dispatches
  battle clicks to the viewer and bombing clicks to the matching
  Reports row.
- i18n keys for the viewer in en + ru.

Docs: ui/docs/battle-viewer-ux.md, FUNCTIONAL.md §6.5 + ru
mirror, ui/PLAN.md Phase 27 decisions + deferred TODOs (push
event, richer class visuals, animated re-distribution).

Tests: Vitest unit (radial layout + timeline frame builder +
marker stroke formula + marker primitives), Playwright e2e for
the viewer (Reports link → viewer, playback step, not-found),
backend engineclient FetchBattle (200 / 404 / bad input), engine
openapi freezes (BattleReport, BattleReportGroup,
BattleActionReport, BattleSummary, Report.battle items).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/engineclient/client.go

@@ -26,6 +26,7 @@ const (
 	pathPlayerCommand   = "/api/v1/command"
 	pathPlayerOrder     = "/api/v1/order"
 	pathPlayerReport    = "/api/v1/report"
+	pathPlayerBattle    = "/api/v1/battle"
 	pathHealthz         = "/healthz"
 )
 
@@ -269,6 +270,41 @@ func (c *Client) GetReport(ctx context.Context, baseURL, raceName string, turn i
 	}
 }
 
+// FetchBattle calls `GET /api/v1/battle/<turn>/<battleID>` and returns
+// the engine response body verbatim alongside the engine status code.
+// 200 carries the BattleReport JSON; 404 means the battle is unknown
+// and the body may be empty. Other 4xx statuses come back wrapped in
+// ErrEngineValidation, everything else in ErrEngineUnreachable.
+func (c *Client) FetchBattle(ctx context.Context, baseURL string, turn int, battleID string) (json.RawMessage, int, error) {
+	if err := validateBaseURL(baseURL); err != nil {
+		return nil, 0, err
+	}
+	if turn < 0 {
+		return nil, 0, fmt.Errorf("engineclient battle get: turn must not be negative, got %d", turn)
+	}
+	if strings.TrimSpace(battleID) == "" {
+		return nil, 0, errors.New("engineclient battle get: battle id must not be empty")
+	}
+	target := baseURL + pathPlayerBattle + "/" + strconv.Itoa(turn) + "/" + url.PathEscape(battleID)
+	body, status, doErr := c.doRequest(ctx, http.MethodGet, target, nil, c.probeTimeout)
+	if doErr != nil {
+		return nil, 0, fmt.Errorf("%w: engine battle get: %w", ErrEngineUnreachable, doErr)
+	}
+	switch status {
+	case http.StatusOK:
+		if len(body) == 0 {
+			return nil, status, fmt.Errorf("%w: engine battle get: empty response body", ErrEngineProtocolViolation)
+		}
+		return json.RawMessage(body), status, nil
+	case http.StatusNotFound:
+		return nil, status, nil
+	case http.StatusBadRequest, http.StatusConflict:
+		return json.RawMessage(body), status, fmt.Errorf("%w: engine battle get: %s", ErrEngineValidation, summariseEngineError(body, status))
+	default:
+		return nil, status, fmt.Errorf("%w: engine battle get: %s", ErrEngineUnreachable, summariseEngineError(body, status))
+	}
+}
+
 // Healthz calls `GET /healthz`. Returns nil on 2xx.
 func (c *Client) Healthz(ctx context.Context, baseURL string) error {
 	if err := validateBaseURL(baseURL); err != nil {

backend/internal/engineclient/client_test.go

@@ -257,6 +257,63 @@ func TestClientGetOrderRejectsBadInput(t *testing.T) {
 	}
 }
 
+func TestClientFetchBattleForwardsPath(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet {
+			t.Fatalf("unexpected method: %s", r.Method)
+		}
+		want := pathPlayerBattle + "/3/" + "11111111-1111-1111-1111-111111111111"
+		if r.URL.Path != want {
+			t.Fatalf("path = %q, want %q", r.URL.Path, want)
+		}
+		_, _ = w.Write([]byte(`{"id":"11111111-1111-1111-1111-111111111111","planet":4}`))
+	}))
+	t.Cleanup(srv.Close)
+
+	cli := newTestClient(t, srv)
+	body, status, err := cli.FetchBattle(context.Background(), srv.URL, 3, "11111111-1111-1111-1111-111111111111")
+	if err != nil {
+		t.Fatalf("FetchBattle: %v", err)
+	}
+	if status != http.StatusOK {
+		t.Fatalf("status = %d", status)
+	}
+	if !strings.Contains(string(body), `"planet":4`) {
+		t.Fatalf("body = %s", body)
+	}
+}
+
+func TestClientFetchBattleNotFound(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	t.Cleanup(srv.Close)
+
+	cli := newTestClient(t, srv)
+	body, status, err := cli.FetchBattle(context.Background(), srv.URL, 0, "11111111-1111-1111-1111-111111111111")
+	if err != nil {
+		t.Fatalf("FetchBattle: %v", err)
+	}
+	if status != http.StatusNotFound {
+		t.Fatalf("status = %d", status)
+	}
+	if body != nil {
+		t.Fatalf("expected nil body on 404, got %s", body)
+	}
+}
+
+func TestClientFetchBattleRejectsBadInput(t *testing.T) {
+	cli := newTestClient(t, httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("server must not be hit on bad input")
+	})))
+	if _, _, err := cli.FetchBattle(context.Background(), "http://example.com", -1, "11111111-1111-1111-1111-111111111111"); err == nil {
+		t.Fatal("expected error on negative turn")
+	}
+	if _, _, err := cli.FetchBattle(context.Background(), "http://example.com", 0, ""); err == nil {
+		t.Fatal("expected error on empty battle id")
+	}
+}
+
 func TestClientHealthzSuccess(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.Path != pathHealthz {