battle-fetch: migrate to user.games.battle ConnectRPC command

The Phase 27 BattleViewer was the last UI surface still issuing raw
fetch() against the backend REST contract (`/api/v1/user/games/...
/battles/...`). The dev-deploy gateway never proxied that path, so
the viewer worked only in tools/local-dev/. Move it onto the signed
ConnectRPC channel every other authenticated surface already uses.

Wire pieces:
- FBS GameBattleRequest in pkg/schema/fbs/battle.fbs, regenerated
  Go + TS bindings.
- MessageTypeUserGamesBattle constant + GameBattleRequest struct in
  pkg/model/report/messages.go.
- pkg/transcoder/battle.go gains GameBattleRequestToPayload and
  PayloadToGameBattleRequest helpers.
- gateway games_commands.go switches on the new message type and
  GETs /api/v1/user/games/{id}/battles/{turn}/{battle_id}; the JSON
  response is re-encoded as a FlatBuffers BattleReport before being
  returned. 404 from backend surfaces as the canonical `not_found`
  gateway error.
- ui/frontend/src/api/battle-fetch.ts now builds the FBS request,
  calls GalaxyClient.executeCommand, and decodes the FBS response
  into the existing UI shape (Record<string,string> race/ship maps,
  string-form UUID). BattleFetchError carries an HTTP-style status
  derived from the result code so the active-view's not_found branch
  keeps working.
- battle.svelte pulls the GalaxyClient from the in-game shell
  context. While the layout's boot Promise.all is in flight the
  effect stays in `loading` until the client handle becomes
  non-null.
- ui/Makefile FBS_INPUTS gains battle.fbs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pkg/model/report/messages.go

@@ -9,6 +9,13 @@ import "github.com/google/uuid"
 // `Report`.
 const MessageTypeUserGamesReport = "user.games.report"
 
+// MessageTypeUserGamesBattle is the authenticated gateway message type
+// used to fetch one battle report through
+// `GET /api/v1/user/games/{game_id}/battles/{turn}/{battle_id}`. The
+// signed payload is a FlatBuffers `GameBattleRequest`; the response is
+// a FlatBuffers `BattleReport`.
+const MessageTypeUserGamesBattle = "user.games.battle"
+
 // GameReportRequest is the typed payload of MessageTypeUserGamesReport.
 // `GameID` selects the target game (the message_type alone is not
 // enough; this scope is per-game) and `Turn` selects the requested
@@ -20,3 +27,19 @@ type GameReportRequest struct {
 	// Turn is the zero-based turn number whose report is requested.
 	Turn uint `json:"turn"`
 }
+
+// GameBattleRequest is the typed payload of MessageTypeUserGamesBattle.
+// `GameID` selects the target game; `Turn` is the turn the battle
+// happened at (the engine partitions battles by turn for cheap lookup);
+// `BattleID` is the in-game identifier returned in the report's
+// battle-summary list. All three fields are required.
+type GameBattleRequest struct {
+	// GameID identifies the game the battle belongs to.
+	GameID uuid.UUID `json:"game_id"`
+
+	// Turn is the turn number the battle happened at.
+	Turn uint `json:"turn"`
+
+	// BattleID is the engine-assigned id of the battle to fetch.
+	BattleID uuid.UUID `json:"battle_id"`
+}

pkg/schema/fbs/battle.fbs

@@ -49,4 +49,14 @@ table BattleReport {
   protocol:[BattleActionReport];
 }
 
+// GameBattleRequest is the signed-gRPC request payload for
+// `MessageTypeUserGamesBattle`. Gateway forwards this into the
+// backend's `GET /api/v1/user/games/{game_id}/battles/{turn}/{battle_id}`
+// endpoint after resolving the caller's runtime player mapping.
+table GameBattleRequest {
+  game_id:UUID (required);
+  turn:uint32;
+  battle_id:UUID (required);
+}
+
 root_type BattleReport;

pkg/schema/fbs/battle/GameBattleRequest.go

@@ -0,0 +1,96 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package battle
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type GameBattleRequest struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsGameBattleRequest(buf []byte, offset flatbuffers.UOffsetT) *GameBattleRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &GameBattleRequest{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishGameBattleRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsGameBattleRequest(buf []byte, offset flatbuffers.UOffsetT) *GameBattleRequest {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &GameBattleRequest{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedGameBattleRequestBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *GameBattleRequest) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *GameBattleRequest) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *GameBattleRequest) GameId(obj *UUID) *UUID {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		x := o + rcv._tab.Pos
+		if obj == nil {
+			obj = new(UUID)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
+func (rcv *GameBattleRequest) Turn() uint32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetUint32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *GameBattleRequest) MutateTurn(n uint32) bool {
+	return rcv._tab.MutateUint32Slot(6, n)
+}
+
+func (rcv *GameBattleRequest) BattleId(obj *UUID) *UUID {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		x := o + rcv._tab.Pos
+		if obj == nil {
+			obj = new(UUID)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
+func GameBattleRequestStart(builder *flatbuffers.Builder) {
+	builder.StartObject(3)
+}
+func GameBattleRequestAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
+	builder.PrependStructSlot(0, flatbuffers.UOffsetT(gameId), 0)
+}
+func GameBattleRequestAddTurn(builder *flatbuffers.Builder, turn uint32) {
+	builder.PrependUint32Slot(1, turn, 0)
+}
+func GameBattleRequestAddBattleId(builder *flatbuffers.Builder, battleId flatbuffers.UOffsetT) {
+	builder.PrependStructSlot(2, flatbuffers.UOffsetT(battleId), 0)
+}
+func GameBattleRequestEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}

pkg/transcoder/battle.go

@@ -382,3 +382,55 @@ func encodeBattleOffsetVector(
 	}
 	return builder.EndVector(length)
 }
+
+// GameBattleRequestToPayload converts model.GameBattleRequest to
+// FlatBuffers bytes suitable for the authenticated gateway transport.
+func GameBattleRequestToPayload(req *model.GameBattleRequest) ([]byte, error) {
+	if req == nil {
+		return nil, errors.New("encode game battle request payload: request is nil")
+	}
+
+	builder := flatbuffers.NewBuilder(64)
+
+	gameHi, gameLo := uuidToHiLo(req.GameID)
+	battleHi, battleLo := uuidToHiLo(req.BattleID)
+
+	fbs.GameBattleRequestStart(builder)
+	fbs.GameBattleRequestAddGameId(builder, fbs.CreateUUID(builder, gameHi, gameLo))
+	fbs.GameBattleRequestAddTurn(builder, uint32(req.Turn))
+	fbs.GameBattleRequestAddBattleId(builder, fbs.CreateUUID(builder, battleHi, battleLo))
+	offset := fbs.GameBattleRequestEnd(builder)
+	fbs.FinishGameBattleRequestBuffer(builder, offset)
+
+	return builder.FinishedBytes(), nil
+}
+
+// PayloadToGameBattleRequest converts FlatBuffers payload bytes into
+// model.GameBattleRequest.
+func PayloadToGameBattleRequest(data []byte) (result *model.GameBattleRequest, err error) {
+	if len(data) == 0 {
+		return nil, errors.New("decode game battle request payload: data is empty")
+	}
+
+	defer func() {
+		if recovered := recover(); recovered != nil {
+			result = nil
+			err = fmt.Errorf("decode game battle request payload: panic recovered: %v", recovered)
+		}
+	}()
+
+	req := fbs.GetRootAsGameBattleRequest(data, 0)
+	gameID := req.GameId(nil)
+	if gameID == nil {
+		return nil, errors.New("decode game battle request payload: game_id is missing")
+	}
+	battleID := req.BattleId(nil)
+	if battleID == nil {
+		return nil, errors.New("decode game battle request payload: battle_id is missing")
+	}
+	return &model.GameBattleRequest{
+		GameID:   uuidFromHiLo(gameID.Hi(), gameID.Lo()),
+		Turn:     uint(req.Turn()),
+		BattleID: uuidFromHiLo(battleID.Hi(), battleID.Lo()),
+	}, nil
+}