fix: parse flat result_summary structure per updated API doc

The API result_summary is a flat object with top-level fields
(total_positions, win_rate, profit_factor, net_pnl, sharpe_ratio, etc.)
not a nested backtest_metadata/instruments map. This was causing all
metrics to parse as None/zero for every completed run.

- Rewrite BacktestResult::from_response() to read flat fields directly
- Replace parse_ratio_value/parse_decimal_str with a single parse_number()
  that accepts both JSON numbers and decimal strings
- Populate winning_positions, losing_positions, total_fees, avg_bars_in_trade
  (previously always None)
- Simplify from_response signature — exchange/base/quote no longer needed
- Add expected_count and coverage_pct to CandleCoverage struct
- Update all example sell rules to use position_quantity instead of "0.01"
- Note that "9999" is a valid sell-all alias (auto-capped by the API)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/prompts.rs

@@ -107,6 +107,8 @@ Buy a fixed number of base units (semantic alias for a decimal string):
 ```json
 {{"kind":"position_quantity"}}
 ```
+Alternatively, `"9999"` works for exits: sell quantities are automatically capped to the open
+position size, so a large fixed number is equivalent to `position_quantity`.
 
 NEVER use placeholder strings like `"ATR_SIZED"`, `"FULL_BALANCE"`, `"all"`, `"dynamic"` —
 these are rejected immediately.
@@ -251,7 +253,7 @@ Common mistakes to NEVER make:
           }}
         ]
       }},
-      "then": {{"side": "sell", "quantity": "0.01"}}
+      "then": {{"side": "sell", "quantity": {{"kind": "position_quantity"}}}}
     }}
   ]
 }}
@@ -302,7 +304,7 @@ Common mistakes to NEVER make:
           }}
         ]
       }},
-      "then": {{"side": "sell", "quantity": "0.01"}}
+      "then": {{"side": "sell", "quantity": {{"kind": "position_quantity"}}}}
     }}
   ]
 }}
@@ -364,7 +366,7 @@ Common mistakes to NEVER make:
           }}
         ]
       }},
-      "then": {{"side": "sell", "quantity": "0.01"}}
+      "then": {{"side": "sell", "quantity": {{"kind": "position_quantity"}}}}
     }}
   ]
 }}
@@ -449,7 +451,7 @@ The MACD line is `EMA(12) - EMA(26)`; the signal line is `EMA(9)` of the MACD li
           }}
         ]
       }},
-      "then": {{"side": "sell", "quantity": "0.01"}}
+      "then": {{"side": "sell", "quantity": {{"kind": "position_quantity"}}}}
     }}
   ]
 }}