feat(V2): /mcp-cross/tools/get_historical with cross-exchange consensus

Add a unified historical endpoint that fans out to every exchange
supporting the requested (asset_class, symbol) pair, then merges the
results into a single consensus candle series with per-bar divergence
metrics:
  - candles[i].close = median across sources
  - candles[i].sources = count of contributing exchanges
  - candles[i].div_pct = (max-min)/median for that bar's close

Crypto routes BTC/ETH/SOL across Bybit + Hyperliquid + Deribit; equities
route to Alpaca for now (IBKR omitted from MVP because its bars endpoint
takes a relative period instead of start/end). Partial failures return a
warning envelope (failed_sources) instead of failing the whole request;
all sources failing → HTTP 502.

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

src/cerbero_mcp/__main__.py

@@ -23,6 +23,7 @@ from cerbero_mcp.exchanges import build_client
 from cerbero_mcp.routers import (
     alpaca,
     bybit,
+    cross,
     deribit,
     hyperliquid,
     ibkr,
@@ -71,6 +72,7 @@ def _make_app(settings: Settings) -> FastAPI:
     app.include_router(ibkr.make_router())
     app.include_router(macro.make_router())
     app.include_router(sentiment.make_router())
+    app.include_router(cross.make_router())
     app.include_router(admin.make_admin_router())
 
     return app