Prevent dangling TAF fragments in city AI reads

City AI can return a partially streamed JSON string when the provider truncates output. The fallback previously kept an unfinished clause such as '但TAF显示', which made the forecast explanation look broken even though earlier evidence was usable.

Constraint: Provider JSON can be truncated after useful fields have already streamed

Rejected: Drop all partial AI text | would lose valid METAR interpretation already returned before truncation

Confidence: high

Scope-risk: narrow

Tested: pytest city AI truncation regression tests

Tested: npm run build

Not-tested: Live DeepSeek provider response

frontend/components/dashboard/scan-terminal/use-ai-city-card-data.ts

@@ -14,7 +14,7 @@ import type { CityDetail, MarketScan } from "@/lib/dashboard-types";
 import { extractStreamingAirportRead } from "./ai-city-stream";
 import { normalizeCityKey } from "./decision-utils";
 
-const AI_CITY_FORECAST_CACHE_PREFIX = "polyWeather_aiCityForecast_v4";
+const AI_CITY_FORECAST_CACHE_PREFIX = "polyWeather_aiCityForecast_v5";
 const AI_CITY_FORECAST_CACHE_TTL_MS = 60 * 60 * 1000;
 const AI_CITY_FORECAST_MAX_CONCURRENT_STREAMS = 2;
 const CITY_MARKET_SCAN_CACHE_PREFIX = "polyWeather_cityMarketScan_v3";
@@ -384,8 +384,8 @@ function buildAiCityFallbackPayload({
   const metarEn = rawMetar
     ? `Latest METAR shows ${currentText}; use it as the live anchor while later reports confirm the path.`
     : `Use ${currentText} and the model path for now while waiting for the next ${bulletinEn}.`;
-  const reasonZh = `DEB、多模型集合和最新${sourceZh}已足够给出当前方向判断；DeepSeek 增强可作为后续补充。`;
-  const reasonEn = `DEB, the model cluster and latest ${sourceEn} are enough for the current directional read; DeepSeek enhancement can be added later.`;
+  const reasonZh = `DEB、多模型集合和最新${sourceZh}已足够给出当前方向判断；页面会在 DeepSeek 返回后合并完整机场报文解读。`;
+  const reasonEn = `DEB, the model cluster and latest ${sourceEn} are enough for the current directional read; the page will merge the full airport-bulletin read when DeepSeek returns.`;
 
   return {
     city_forecast: {