fix(ton-trading-bot): use exit price for simulation balance P&L conversion (issue #133)

[konard]

Summary

Fixes the simulation balance P&L calculation in ton_trading_record_trade where profit was being incorrectly reported for TON→USDT trades with significant price appreciation.

Root cause: When crediting back the simulation balance on trade close, the USD P&L was divided by the entry price (USD/TON at trade open) instead of the exit price (current USD/TON rate). For a +10.6% TON price move ($1.26→$1.393), this over-credits the balance by 10.6%, yielding ~1.583 TON profit when the correct value is ~1.432 TON.

Fix: Use exit_price_usd (falling back to entry_price_usd when not provided) to convert USD P&L back to TON units. This correctly reflects how much TON the USD profit can buy at the current market rate.

- const entryTonPriceUsd = entry.entry_price_usd ?? null;
- const creditTon = entryTonPriceUsd != null
-   ? entry.amount_in + pnl / entryTonPriceUsd   // ← used entry price (wrong)
+ const exitTonPriceUsd = exit_price_usd ?? entry.entry_price_usd ?? null;
+ const creditTon = exitTonPriceUsd != null
+   ? entry.amount_in + pnl / exitTonPriceUsd    // ← uses exit price (correct)
    : amount_out;

Verification (issue #133 example):

	Formula	Result
Buggy (entry $1.26)	15 + $1.995 / $1.26	+1.583 TON (over-credited)
Fixed (exit $1.393)	15 + $1.995 / $1.393	+1.432 TON ≈ ~1.43 TON ✓

Test plan

• New regression test: credits back correct profit for large price move (issue #133): uses exit_price not entry_price — reproduces the exact scenario from the issue (15 TON, $1.26→$1.393) and asserts the balance credit uses exit price
• Updated 2 existing balance-credit tests whose expected values were based on the wrong (entry-price) formula
• All 268 tests pass locally (npm test)

Closes #133

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[konard]

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[konard]

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