feat: weighted random routing + mode switching + EWMA smoothing + stale eviction

- Weighted random routing (math/rand/v2 per-goroutine ChaCha8)
- PUT /api/route/mode for runtime mode switch (best / weighted_random)
- EWMA score smoothing (configurable smoothing_factor, default 0.3)
- Periodic stale node eviction (5 min threshold, 60s interval)
- Non-blocking WS broadcast (per-client buffered channel + writePump)
- Background rate limiter cleanup goroutine
- In-memory trunk gateway cache (zero disk I/O in hot path)
- Configurable load_avg multiplier (default 50.0, backward compat)
- UI mode indicator + admin Switch button in dashboard
- 42 tests: weighted random, mode switching, EWMA, eviction, API integration

internal/models/types.go

@@ -226,6 +226,7 @@ type BalancerInfo struct {
 	RouteTotal     int64  `json:"route_total"`
 	RouteFallbacks int64  `json:"route_fallbacks"`
 	UptimeSec      int64  `json:"uptime_sec"`
+	BalanceMode    string `json:"balance_mode"`
 }
 
 // --- WebSocket ---