[SECURITY] Fix critical vault drain vulnerability in partial fill logic

[byte541]

Summary

This PR fixes a critical security vulnerability that allows attackers to drain the market vault by exploiting incorrect amount recording in partial fill events.

Vulnerability Details

Severity: CRITICAL
Type: Vault Drain / Theft of Funds

When a maker's order is partially filled, the Fill event incorrectly records base_amount_to_set (the remaining unfilled amount) instead of the actual amount that was matched.

Vulnerable Code (before fix)

market_events.add_event(EventParams{
    base_amount: order.base_amount_to_set,  // BUG: This is REMAINING, not MATCHED
    ...
});

Impact

1. Attacker places large Bid order (e.g., 10,000 tokens)
2. Accomplice places small Ask order (e.g., 1 token)
3. Partial fill: 1 token matched, but event records 9,999 (remaining)
4. consume_events credits attacker with 9,999 tokens instead of 1
5. Attacker settles and drains 9,999 tokens from vault

Fix

Added base_amount_matched field to track actual matched amount and use it in Fill events:

struct EditOrders {
    pub order_id: u64,
    pub base_amount_to_set: u64,
    pub base_amount_matched: u64,  // NEW: Track actual matched amount
    pub total_quote_amount: u64
}

// In event creation:
market_events.add_event(EventParams{
    base_amount: order.base_amount_matched,  // FIX: Use matched amount
    ...
});

Testing

I have prepared a proof-of-concept test demonstrating the vulnerability and fix. Happy to share if helpful.

Disclosure

This vulnerability was discovered during security research. I'm submitting this fix as part of responsible disclosure.

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Found by byte541