feat: add declarative SizingMethod types from upstream schema

Upstream added three new quantity sizing objects alongside DecimalString and Expr:
- fixed_sum: buy N quote-currency worth at current price
- percent_of_balance: buy N% of named asset's free balance
- fixed_units: buy exactly N base units (semantic alias for decimal string)

Update dsl-schema.json to include the three definitions and expand
Action.quantity.oneOf to reference all five valid forms.

Update prompts.rs Quantity section to present the declarative methods
as the preferred approach — they're cleaner, more readable, and
instrument-agnostic compared to raw Expr composition.

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

src/prompts.rs

@@ -74,36 +74,42 @@ bars_since_entry — complete bars elapsed since position was opened
 balance — free balance of a named asset (e.g. "usdt", "usdc")
 
 ### Quantity
-Action quantity is either a fixed decimal string **or** an Expr that evaluates to a number
-at candle close. If the Expr returns None the order is skipped; negative values are clamped
-to zero.
+Action quantity accepts four forms — pick the simplest one for your intent:
 
-**Preferred: expression-based sizing** — instrument-agnostic, scales automatically:
+**1. Declarative sizing methods (preferred — instrument-agnostic, readable):**
 
-Fixed fraction of quote balance (5% of USDC balance, converted to base units at current price):
+Spend a fixed quote amount (e.g. $500 worth of base at current price):
 ```json
-{{"kind":"bin_op","op":"div",
-  "left":{{"kind":"bin_op","op":"mul",
-    "left":{{"kind":"balance","asset":"usdc"}},
-    "right":{{"kind":"literal","value":"0.05"}}}},
-  "right":{{"kind":"field","field":"close"}}}}
+{{"method":"fixed_sum","amount":"500"}}
 ```
 
-ATR-based risk sizing (risk $200 per trade, sized by volatility):
+Spend a percentage of free quote balance (e.g. 5% of USDC):
+```json
+{{"method":"percent_of_balance","percent":"5","asset":"usdc"}}
+```
+
+Buy a fixed number of base units (semantic alias for a decimal string):
+```json
+{{"method":"fixed_units","units":"0.01"}}
+```
+
+**2. Plain decimal string** — use only when you have a specific reason:
+`"0.01"` (0.01 BTC, 3.0 ETH, 50.0 SOL — instrument-specific, not portable)
+
+**3. Expr** — for dynamic sizing not covered by the methods above, e.g. ATR-based:
 ```json
 {{"kind":"bin_op","op":"div",
   "left":{{"kind":"literal","value":"200"}},
   "right":{{"kind":"func","name":"atr","period":14}}}}
 ```
 
-For exit rules, use `position_quantity` to close the exact open position:
+**4. Exit rules** — use `position_quantity` to close the exact open size:
 ```json
 {{"kind":"position_quantity"}}
 ```
 
-**Fixed strings are also valid** when you want a specific size, e.g. `"0.01"`.
 NEVER use placeholder strings like `"ATR_SIZED"`, `"FULL_BALANCE"`, `"all"`, `"dynamic"` —
-these are rejected immediately. Use an Expr or a plain decimal string.
+these are rejected immediately.
 
 ### Multi-timeframe
 Any expression can reference a different timeframe via "timeframe" field.