feat: Tradier broker mirror — config-switch redundancy for Alpaca execution

tradier_broker/ mirrors every trading call the live loop makes on
AlpacaPaperClient (account, clock, positions, orders, submit incl.
bracket->OTOCO, cancel, replace, close) with identical signatures and
Alpaca-shaped returns, so trading_execution / exit_manager / flatten /
execution / sync run unchanged. Market data always delegates to Alpaca.

- BROKER=tradier (+ TRADIER_* env) swaps execution client in run_live_paper;
  default path untouched
- client_order_id -> tag preserves the lost-submit reconcile
- TradierApiError subclasses AlpacaApiError so the executor's
  rejection-vs-transport fork works as-is
- sandbox-verified: balances/clock/positions/orders, OTOCO preview
  (result:true), place -> tag lookup -> cancel round trip
- 18 unit tests (translation + client paths); executor integration checked
- docs/TRADIER-FALLBACK.md switch runbook

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tradier_broker/smoke.py

@@ -0,0 +1,95 @@
+"""Tradier sandbox smoke test — proves the mirror talks to the broker.
+
+Read-only by default: balances, clock, positions, orders. Optional order
+exercises (sandbox-safe):
+
+  python -m tradier_broker.smoke                    # read-only checks
+  python -m tradier_broker.smoke --preview AAPL     # preview (never submits) a
+                                                    # bracket + a market order
+  python -m tradier_broker.smoke --place-cancel AAPL  # place a far-below-market
+                                                      # limit buy, then cancel it
+
+Requires TRADIER_ACCESS_TOKEN + TRADIER_ACCOUNT_ID in env/.env.
+Refuses order exercises against the live base URL.
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+import time
+
+from dotenv import load_dotenv
+
+from tradier_broker.client import TradierClient, build_order_form
+from tradier_broker.settings import TradierSettings
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--preview", metavar="SYMBOL",
+                        help="preview (validate, never submit) a bracket and a market order")
+    parser.add_argument("--place-cancel", metavar="SYMBOL",
+                        help="place a $0.50-limit buy for 1 share, then cancel it")
+    args = parser.parse_args(argv)
+
+    load_dotenv()
+    settings = TradierSettings.from_env()
+    if not settings.is_configured:
+        print("TRADIER_ACCESS_TOKEN / TRADIER_ACCOUNT_ID not set")
+        return 1
+    print(f"env: {'SANDBOX' if settings.is_sandbox else 'LIVE'}  "
+          f"account: {settings.account_id}")
+    client = TradierClient(settings)
+
+    account = client.get_account()
+    print(f"account: equity={account['equity']} cash={account['cash']} "
+          f"buying_power={account['buying_power']}")
+    clock = client.get_clock()
+    print(f"clock: state={clock['state']} is_open={clock['is_open']}")
+    positions = client.get_positions(fresh=True)
+    print(f"positions: {len(positions)}")
+    for p in positions:
+        print(f"  {p['symbol']} qty={p['qty']} avg={p['avg_entry_price']} "
+              f"last={p['current_price']} upl={p['unrealized_pl']}")
+    orders = client.get_orders(status="all")
+    print(f"orders (session): {len(orders)}")
+    for o in orders[:10]:
+        print(f"  {o['id']} {o['symbol']} {o['side']} {o['qty']} {o['type']} "
+              f"status={o['status']} tag={o['client_order_id'] or '-'}")
+
+    if (args.preview or args.place_cancel) and not settings.is_sandbox:
+        print("refusing order exercises against LIVE base URL")
+        return 1
+
+    if args.preview:
+        sym = args.preview.upper()
+        for label, kw in (
+            ("bracket limit", dict(order_type="limit", limit_price=1.00,
+                                   stop_loss_price=0.80, take_profit_price=1.50)),
+            ("plain market", dict(order_type="market")),
+        ):
+            form = build_order_form(sym, 1, "buy", kw.pop("order_type"), "day",
+                                    kw.get("limit_price"), None,
+                                    kw.get("stop_loss_price"),
+                                    kw.get("take_profit_price"), None)
+            form["preview"] = "true"
+            payload = client._acct("POST", "/orders", form=form)  # noqa: SLF001
+            print(f"preview {label}: {json.dumps(payload)[:400]}")
+
+    if args.place_cancel:
+        sym = args.place_cancel.upper()
+        resp = client.submit_order(symbol=sym, qty=1, side="buy",
+                                   order_type="limit", limit_price=0.50,
+                                   client_order_id=f"smoke-{int(time.time())}")
+        print(f"placed: id={resp['id']} status={resp['status']}")
+        found = client.get_order_by_client_order_id(resp["client_order_id"])
+        print(f"lookup by tag: {'FOUND ' + str(found['status']) if found else 'NOT FOUND'}")
+        client.cancel_order(resp["id"])
+        print("cancelled ok")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())