Add T-shaped options chain view with straddle metrics (phase A).

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/options/options_pricing_lib.py

@@ -358,3 +358,49 @@ def equivalent_contract_leverage(
     if eth_amount <= 0 or total_premium <= 0:
         return None
     return round(float(index_px) * float(eth_amount) / float(total_premium), 1)
+
+
+def straddle_ask_per_unit(
+    call_ask: float | None,
+    put_ask: float | None,
+) -> float | None:
+    """跨式双买:每 1 标的币的卖一报价之和."""
+    if call_ask is None or put_ask is None:
+        return None
+    if float(call_ask) <= 0 or float(put_ask) <= 0:
+        return None
+    return round(float(call_ask) + float(put_ask), 4)
+
+
+def straddle_premium_total(
+    call_ask: float | None,
+    put_ask: float | None,
+    eth_amount: float | None,
+) -> float | None:
+    """跨式双买权利金总额(USDC)."""
+    per = straddle_ask_per_unit(call_ask, put_ask)
+    if per is None or eth_amount is None or float(eth_amount) <= 0:
+        return None
+    return round(per * float(eth_amount), 2)
+
+
+def straddle_breakeven_band(
+    strike: float | None,
+    combined_ask_per_unit: float | None,
+) -> tuple[float | None, float | None]:
+    """跨式到期平衡带:下平衡 ~ 上平衡(按双卖一报价和)."""
+    if strike is None or combined_ask_per_unit is None:
+        return None, None
+    k = float(strike)
+    d = float(combined_ask_per_unit)
+    return round(k - d, 2), round(k + d, 2)
+
+
+def format_straddle_band(
+    strike: float | None,
+    combined_ask_per_unit: float | None,
+) -> str:
+    lo, hi = straddle_breakeven_band(strike, combined_ask_per_unit)
+    if lo is None or hi is None:
+        return ""
+    return f"{lo:.0f} ~ {hi:.0f}"