feat: add reverse flag and symmetric short support to DSL

Update scout's schema and system prompt to reflect two upstream swym
changes from 2026-03-10:

- b535207: symmetric short quantity fix — buy-to-cover now correctly
  uses position_qty (executor was broken; scout's DSL patterns were
  already correct and will now work as intended)

- 6f58949: reverse flag on Action — new optional "reverse": true field
  that submits position_qty + configured_qty when an opposite position
  is open, closing it and opening a new one in the opposite direction
  in a single order (flip-through-zero)

Changes:
- dsl-schema.json: add "reverse" boolean to Action definition
- prompts.rs: add "Reverse / flip-through-zero" capability section
  and Example 6 (2-rule EMA flip strategy) to FUTURES_SHORT_EXAMPLES

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

src/dsl-schema.json

@@ -74,6 +74,11 @@
             { "$ref": "#/definitions/SizingFixedUnits" },
             { "$ref": "#/definitions/Expr" }
           ]
+        },
+        "reverse": {
+          "type": "boolean",
+          "default": false,
+          "description": "Flip-through-zero flag (futures only). When true and an opposite position is currently open, the submitted order quantity becomes position_qty + configured_qty, closing the existing position and immediately opening a new one in the opposite direction in a single order. When flat the flag has no effect and configured_qty is used as normal. Omit or set false for standard close-only behaviour."
         }
       }
     },

src/prompts.rs

@@ -121,6 +121,24 @@ CRITICAL — the `"method"` vs `"kind"` distinction:
   `multiplier` field. Only `amount` is accepted alongside `method`.
 - NEVER add extra fields to SizingMethod objects — they use `additionalProperties: false`.
 
+### Reverse / flip-through-zero (futures only)
+
+Setting `"reverse": true` on a rule action enables a single-order position flip on futures.
+When an opposite position is open, quantity = `position_qty + configured_qty`, which closes
+the existing position and opens a new one in the opposite direction in one order (fees split
+proportionally). When flat the flag has no effect — `configured_qty` is used normally.
+
+This lets you collapse a 4-rule long+short strategy (separate open/close for each leg) into
+2 rules, reducing round-trip fees and keeping logic compact:
+
+```json
+{{"side": "sell", "quantity": {{"method": "percent_of_balance", "percent": "10", "asset": "usdc"}}, "reverse": true}}
+```
+
+Use `reverse` when you always want to be in a position — the signal flips you from long to
+short (or vice versa) rather than first exiting and then re-entering separately. Do NOT use
+`reverse` on spot markets (short selling is not supported there).
+
 ### Multi-timeframe
 Any expression can reference a different timeframe via "timeframe" field.
 Use higher timeframes as trend filters, lower timeframes for entry precision.
@@ -557,7 +575,58 @@ Key short-specific notes:
 - Stop-loss for short = close > entry_price * (1 + stop_pct), e.g. `* 1.02` for 2% stop
 - Take-profit for short = close < entry_price * (1 - target_pct), e.g. `* 0.97` for 3% target
 - Short exit uses `"side": "buy"` with `{"kind": "position_quantity"}` (same as long exit uses sell)
-- `percent_of_balance` for short entry uses `"usdc"` as the asset (the collateral currency)"##;
+- `percent_of_balance` for short entry uses `"usdc"` as the asset (the collateral currency)
+
+### Example 6 — Futures flip-through-zero: 2-rule EMA trend-follower using `reverse`
+
+When you always want to be in a position (long during uptrends, short during downtrends),
+use `"reverse": true` to flip from one side to the other in a single order. This uses half
+the round-trip fee count compared to a 4-rule separate-entry/exit approach.
+
+```json
+{
+  "type": "rule_based",
+  "candle_interval": "4h",
+  "rules": [
+    {
+      "comment": "Go long (or flip short→long): EMA9 crosses above EMA21 while above EMA50",
+      "when": {
+        "kind": "all_of",
+        "conditions": [
+          {"kind": "any_of", "conditions": [
+            {"kind": "position", "state": "flat"},
+            {"kind": "position", "state": "short"}
+          ]},
+          {"kind": "ema_crossover", "fast_period": 9, "slow_period": 21, "direction": "above"},
+          {"kind": "ema_trend", "period": 50, "direction": "above"}
+        ]
+      },
+      "then": {"side": "buy", "quantity": {"method": "percent_of_balance", "percent": "10", "asset": "usdc"}, "reverse": true}
+    },
+    {
+      "comment": "Go short (or flip long→short): EMA9 crosses below EMA21 while below EMA50",
+      "when": {
+        "kind": "all_of",
+        "conditions": [
+          {"kind": "any_of", "conditions": [
+            {"kind": "position", "state": "flat"},
+            {"kind": "position", "state": "long"}
+          ]},
+          {"kind": "ema_crossover", "fast_period": 9, "slow_period": 21, "direction": "below"},
+          {"kind": "ema_trend", "period": 50, "direction": "below"}
+        ]
+      },
+      "then": {"side": "sell", "quantity": {"method": "percent_of_balance", "percent": "10", "asset": "usdc"}, "reverse": true}
+    }
+  ]
+}
+```
+
+Key flip-strategy notes:
+- Gate each rule on `flat OR opposite` (using `any_of`) so it fires both on initial entry and on flip
+- `reverse: true` handles the flip math automatically — no need to size for `position_qty + new_qty`
+- This pattern works best for trend-following where you want continuous market exposure
+- Still add a time-based or ATR stop if you want a safety exit when the trend stalls"##;
 
 /// Build the user message for the first iteration (no prior results).
 /// `prior_summary` contains a formatted summary of results from previous runs, if any.