fix: HL size floor, random scan jitter 50-250ms

- GetHLSize: change rounding from Sprintf (round-to-nearest)
  to math.Floor (floor), consistent with GetBitgetSize
- Scan interval: fixed 200ms → random 50-250ms to avoid
  lock-step with HyperLiquid's ~200ms allMids push cycle
- README: update architecture diagram, trading logic, config note

README.md

@@ -19,7 +19,8 @@ PriceStore ─────────┼─────────────
                     └──────────────┘
                            │
                  ┌─────────▼─────────┐
-                 │ ScanBGHL (200ms)   │
+                 │ ScanBGHL (50-250ms │
+                 │   random jitter)   │
                  │ BG ↔ HL only       │
                  └─────────┬─────────┘
                            │
@@ -92,11 +93,13 @@ Then open [http://localhost:8888](http://localhost:8888) for the Web dashboard.
 | `TRADE_THRESHOLD` | `0.15` | Min net profit % to enter (after fees) |
 | `TRADE_AMOUNT_USD` | `10` | USD per leg |
 | `TRADE_COOLDOWN_MS` | `30000` | Cooldown between same-coin trades (ms) |
-| `TEST_MODE` | `false` | Simulate orders (no real API calls) |
+| `TEST_MODE` | `false` | Simulate orders with mock fills (no real API calls) |
 | `MOCK_SLIPPAGE_PCT` | `0.005` | Simulated slippage per leg (%) |
 | `BITGET_API_KEY` / `BITGET_API_SECRET` / `BITGET_PASSPHRASE` | — | Bitget API credentials (test mode skips) |
 | `HL_PRIVATE_KEY` / `HL_ADDRESS` | — | HyperLiquid wallet credentials (test mode skips) |
 
+> **Note:** Scan interval is fixed at **50-250ms random jitter** (not configurable). This prevents lock-step with HyperLiquid's ~200ms push cycle.
+
 ## Fee Model
 
 All trades use **maker** (limit orders), no rebate. Only Bitget and HyperLiquid are used for trading:
@@ -110,7 +113,7 @@ Round trip (2 legs entry + 2 legs exit): **0.07%** total fees.
 
 ## Trading Logic
 
-1. **Scanner** runs every 500ms, checks all 6 coins for BG ↔ HL spread
+1. **Scanner** runs every 50-250ms (random jitter to avoid lock-step with exchange push cycles), checks all 6 coins for BG ↔ HL spread
 2. **Entry** when net profit ≥ `TRADE_THRESHOLD` (after full round-trip fees)
    - Uses scan-time prices directly (no re-read from store to avoid WS jitter)
    - Synchronous execution in the scanner tick (no goroutine delay)