feat(web,coder): arena pane — compare 2-6 AI competitors on same prompt

Arena is a new pane kind for competitive AI evaluation. A Battle runs
the same prompt against 2-6 Contestants across two concurrent lanes:
local lane (llama-swap models, serial) and cloud lane (parallel).

Added to all three registries: @boocode/contracts WsFrameSchema,
server InferenceFrame, and web WsFrame.

Backend (apps/coder):
- arena-runner: battle scheduler, lane classifier, benchmark, results
  writer, resume, user winner override
- arena-analyzer: two-stage digest→judge analysis on DEFAULT_MODEL
- arena-decisions: status transitions and resume logic (unit-tested)
- arena-analyzer-helpers: pure helper functions (unit-tested)
- arena-model-call: model call utility for analysis
- arena routes: create/get/list/stop/analyze/cross-examine/winner/diff
- schema: battles, contestants, cross_examinations tables (idempotent)
- remove old /api/arena* routes and tasks.arena_id column

Frontend (apps/web):
- ArenaLauncherDialog: battle type, prompt, contestant selection
- ArenaPane: live roster, streaming output, analysis, cross-exam
- DiffView: unified diff with line-by-line color for coding contests
- Winner override per-row dropdown (Trophy icon)
- battle_updated WS handler for live winner/analysis updates
- arena pane kind in Workspace, ChatTabBar, useSidebar

Cross-app:
- ArenaState and ArenaContestantShape/WsFrame types (contracts)
- battle_* frames in WsFrameSchema, InferenceFrame, and web WsFrame
- manifest.json written per battle results folder
- /Arena added to .gitignore

apps/web/src/hooks/useWorkspacePanes.ts

@@ -3,6 +3,7 @@ import type { DragEvent } from 'react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
 import type {
+  ArenaState,
   ClosedPaneEntry,
   HtmlArtifactState,
   MarkdownArtifactState,
@@ -187,6 +188,16 @@ function orchestratorPane(state: OrchestratorState): WorkspacePane {
   };
 }
 
+function arenaPane(state: ArenaState): WorkspacePane {
+  return {
+    id: generateId(),
+    kind: 'arena',
+    chatIds: [],
+    activeChatIdx: -1,
+    arena_state: state,
+  };
+}
+
 // v1.9: settings panes are ephemeral. Filter them out before persisting so a
 // page reload always returns to a clean workspace; the user re-opens via the
 // sidebar Settings button when needed.
@@ -290,6 +301,8 @@ export interface UseWorkspacePanesResult {
   createTab: (paneIdx: number, kind: WorkspaceTabKind) => Promise<void>;
   /** Open an orchestrator run pane (or focus an existing one for the same run_id). */
   addOrchestratorPane: (state: OrchestratorState) => string | null;
+  /** Open an arena battle pane (or focus an existing one for the same battle_id). */
+  addArenaPane: (state: ArenaState) => string | null;
   /** Back-compat alias for createTab(paneIdx, 'coder'). */
   createCoderTab: (paneIdx: number) => Promise<void>;
   // Open-on-first-click, close-on-second-click. Singleton — settings panes
@@ -877,6 +890,38 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     });
   }, [addOrchestratorPane]);
 
+  const addArenaPane = useCallback((state: ArenaState): string | null => {
+    let openedId: string | null = null;
+    setPanes((prev) => {
+      const existingIdx = prev.findIndex(
+        (p) => p.kind === 'arena' && p.arena_state?.battle_id === state.battle_id,
+      );
+      if (existingIdx >= 0) {
+        setActivePaneIdx(existingIdx);
+        openedId = prev[existingIdx]!.id;
+        return prev;
+      }
+      if (nonSettingsCount(prev) >= MAX_PANES) {
+        toast.error(`Maximum ${MAX_PANES} panes`);
+        return prev;
+      }
+      const newPane = arenaPane(state);
+      openedId = newPane.id;
+      const next = [...prev, newPane];
+      setActivePaneIdx(next.length - 1);
+      return next;
+    });
+    return openedId;
+  }, []);
+
+  // Arena pane: open via sessionEvents (fired by the launcher).
+  useEffect(() => {
+    return sessionEvents.subscribe((ev) => {
+      if (ev.type !== 'open_arena_pane') return;
+      addArenaPane(ev.state);
+    });
+  }, [addArenaPane]);
+
   // Returns the new settings pane id when one is OPENED (so mobile callers can
   // push ?pane= atomically — see addPaneAndSwitch), or null when it was closed.
   // Id generated outside the updater so a strict-mode double-invoke agrees.
@@ -1121,6 +1166,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     addSplitPane,
     createTab,
     addOrchestratorPane,
+    addArenaPane,
     createCoderTab,
     toggleSettingsPane,
     removePane,