feat: client-side validation, cycling detection, quantity prompt fix

- validate_strategy(): hard error if quantity is not a parseable decimal
  (catches "ATR_SIZED" etc. before sending to swym API); soft warning if
  a sell rule has no entry_price stop-loss or no bars_since_entry time exit
- Hard validation errors skip the backtest and feed errors back to the LLM
  via IterationRecord.validation_notes included in summary()
- json_contains_kind(): recursive helper to search strategy JSON tree
- diagnose_history(): add cycling detection — triggers is_converged when
  any avg_sharpe value appears 3+ times in history (not just last 3 streak),
  catching the alternating RSI<30 / RSI<25 pattern seen in the latest run
- prompts: clarify that quantity must parse as a float; list invalid
  placeholder strings ("ATR_SIZED", "FULL_BALANCE", "dynamic", etc.)

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

src/prompts.rs

@@ -51,8 +51,11 @@ bars_since_entry — complete bars elapsed since position was opened
 balance — free balance of a named asset (e.g. "usdt", "usdc")
 
 ### Quantity
-Action quantity MUST be a fixed decimal string, e.g. `"quantity": "0.001"`.
+Action quantity MUST be a fixed decimal string that parses as a floating-point number,
+e.g. `"quantity": "0.001"`.
 NEVER use an expression object for quantity — only plain decimal strings are accepted.
+NEVER use placeholder strings like `"ATR_SIZED"`, `"FULL_BALANCE"`, `"percent_of_balance"`,
+`"dynamic"`, or any non-numeric string — these will be rejected immediately.
 
 ### Multi-timeframe
 Any expression can reference a different timeframe via "timeframe" field.
@@ -314,6 +317,7 @@ Common mistakes to NEVER make:
 - Always gate buy rules with position state "flat" and sell rules with "long"
 - Never add a short-entry (sell when flat) rule — spot markets are long-only
 - Never use an expression object for `quantity` — it must always be a plain decimal string like `"0.001"`
+- Never use a placeholder string for `quantity` — `"ATR_SIZED"`, `"FULL_BALANCE"`, `"dynamic"`, etc. are all invalid and will be rejected. Use `"0.001"` or similar.
 "##
     )
 }