feat: position sizing follows the strategy config — default 4x equity notional

Positions were stuck at ~equity×1.2 (~$420 on a $500 account) no matter
what the strategy said: both the manager load path and the per-cycle config
reload hardcoded vergex strategies to a 6-position × equity×1.2 book,
silently overriding the configured 10x ratio. Both overrides removed —
sizing now comes from the strategy's own RiskControl (ClampLimits bounds it,
margin auto-reduce keeps the book solvent).

Defaults aligned across all three config sources (backend default template,
quick-create preset, studio unified config): 2 positions × 4x equity
notional at 10x leverage ≈ 80% margin when both are open — concentrated per
the operator's request. Live strategy updated via relaunch; loader log
confirms maxPos=2 posRatio=4.0.

web/src/pages/StrategyStudioPage.tsx

@@ -1391,8 +1391,10 @@ export function StrategyStudioPage() {
         max_positions: 2,
         btc_eth_max_leverage: 10,
         altcoin_max_leverage: 10,
-        btc_eth_max_position_value_ratio: 10,
-        altcoin_max_position_value_ratio: 10,
+        // 4× equity notional per position — at 10x leverage two full
+        // positions use ~80% of margin (concentrated but solvent)
+        btc_eth_max_position_value_ratio: 4,
+        altcoin_max_position_value_ratio: 4,
         max_margin_usage: 1.0,
         min_confidence: 78,
         min_risk_reward_ratio: 3,