Fix: Prevent negative plus balance from unfiltered Scrypt pending amounts

[TaprootFreak]

Problem

Critical bug causing negative plus balances in financial logs, particularly affecting Scrypt/EUR (Asset 401):

• Log 1304749 (09:12): plusBalance = -201,499 EUR ❌
• Log 1304742 (09:09): plusBalance = -29,999 EUR ❌
• Expected: Plus balances should never be negative

Impact

• plusBalanceChf stored as NULL in logs (rejected by getJsonValue() for negative values)
• Total balance calculations corrupted
• Financial type summations failed (EUR had no plusBalanceChf field)
• System-wide balance tracking compromised

Root Cause

// Line 719-726: Unfiltered values calculated
let fromScryptUnfiltered = pendingChf... + pendingEur... + pendingScrypt...;
let toScryptUnfiltered = pendingBank... + pendingChf... + pendingEur...;

// Line 796-804: Added to totalPlusPending WITHOUT clamping
const totalPlusPending = ... + 
  (useUnfilteredTx ? fromScryptUnfiltered : fromScrypt) +  // ← Can be negative!
  (useUnfilteredTx ? toScryptUnfiltered : toScrypt);       // ← Can be negative!

// Line 806: Used in totalPlus calculation
const totalPlus = liquidity + totalPlusPending + ...;  // ← Goes negative!

Filtered versions (fromScrypt, toScrypt) were clamped to 0 if negative (lines 775-793).
Unfiltered versions were NOT clamped → negative values propagated to totalPlus.

Solution

1. Changed fromScryptUnfiltered and toScryptUnfiltered from const to let
2. Added clamping logic after line 793:

   if (fromScryptUnfiltered < 0) {
     errors.push(`fromScryptUnfiltered < 0`);
     this.logger.verbose(...);
     fromScryptUnfiltered = 0;
   }
   
   if (toScryptUnfiltered < 0) {
     errors.push(`toScryptUnfiltered < 0`);
     this.logger.verbose(...);
     toScryptUnfiltered = 0;
   }

Testing

• ✅ All existing tests pass
• ✅ Lint passed
• ✅ Build successful
• Prevents negative totalPlusPending from corrupting plus balances
• Maintains error logging for debugging

Verification

Used diagnostic scripts (PR #3329) to identify the issue:

• compare-balance-logs.sh: Found Scrypt asset changes
• sum-asset-balances.sh: Showed EUR plusBalanceChf = -114k CHF
• inspect-asset-balance.sh: Revealed totalPlus = -201k, liquidity = 1, pending = null

Related

• Affects all Scrypt assets when unfiltered pending amounts are used
• Most visible on Scrypt/EUR due to high transaction volume
• Similar pattern exists for Kraken but was not observed as negative