fix: two Bollinger Band DSL errors from 50-iteration run

- bollinger_upper/lower func Exprs must NOT include a "field" parameter;
  they compute from close internally. Setting "field":"bollinger_upper"
  causes API rejection: expected one of open/high/low/close/volume.
- bollinger Condition "band" only accepts "above_upper" or "below_lower";
  "above_lower" and "below_upper" are invalid variants.

Both errors appeared repeatedly across the 50-iteration run, causing
failed backtest submissions on every Bollinger crossover strategy.

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

src/prompts.rs

@@ -162,6 +162,11 @@ Common mistakes to NEVER make:
 - `bollinger_upper` and `bollinger_lower` are FUNC NAMES, not Expr kinds. To compare close to the upper band:
   `{{"kind":"compare","left":{{"kind":"field","field":"close"}},"op":">","right":{{"kind":"func","name":"bollinger_upper","period":20}}}}`
   NEVER write `{{"kind":"bollinger_upper",...}}` — `bollinger_upper` is not an Expr kind.
+  NEVER set `"field":"bollinger_upper"` on a func Expr — `bollinger_upper`/`bollinger_lower` have no `field`
+  parameter; they compute from close internally. Just `{{"kind":"func","name":"bollinger_upper","period":20}}`.
+- The `{{"kind":"bollinger",...}}` Condition (shorthand) only accepts `"band": "above_upper"` or
+  `"band": "below_lower"`. There is NO `above_lower` or `below_upper` — those are invalid and will be
+  rejected. Use `above_upper` (price above the upper band) or `below_lower` (price below the lower band).
 - `adx` is a FUNC NAME, not a Condition kind. To filter for strong trends (ADX > 25):
   `{{"kind":"compare","left":{{"kind":"func","name":"adx","period":14}},"op":">","right":{{"kind":"literal","value":"25"}}}}`
   NEVER write `{{"kind":"adx",...}}` — `adx` is not a Condition kind, it is a FuncName used inside `{{"kind":"func",...}}`.