fix: include total_returned in FlushToInsurance available-balance guard

[0x-SquidSol]

Summary

process_flush_to_insurance computed the available-to-flush balance as:

available = total_deposited - total_withdrawn - total_flushed

but omitted + total_returned.

Impact

After AdminWithdrawInsurance runs (incrementing total_returned), the vault physically holds those returned tokens again, but the old formula still subtracted the full total_flushed without adding back total_returned. This causes:

1. Incorrect capacity reporting — valid flushes are rejected even when tokens are present in the vault.
2. Spurious Overflow — if total_flushed > total_deposited - total_withdrawn after a partial return cycle, checked_sub returns None → Err(Overflow), permanently blocking FlushToInsurance even though the vault is solvent.

A pool that has completed one flush+return cycle can never re-flush insurance capital without triggering this error, locking the admin out of a core operational action.

Fix

Add .checked_add(pool.total_returned) to match the formula already used by total_pool_value():

let available = pool.total_deposited
    .checked_sub(pool.total_withdrawn)
    .and_then(|v| v.checked_sub(pool.total_flushed))
    .and_then(|v| v.checked_add(pool.total_returned))  // ← added
    .ok_or(StakeError::Overflow)?;

Test plan

• cargo test passes (57 tests)
• Manual trace: deposited=1000, withdrawn=0, flushed=1000, returned=500 → old formula: underflow → new formula: available=500 ✓

🤖 Generated with Claude Code

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Configuration used: defaults

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Run ID: 5881edce-0a2f-46f6-a9de-436052aa4797

📥 Commits

Reviewing files that changed from the base of the PR and between 1881738 and d66a87d.

📒 Files selected for processing (1)

• src/processor.rs

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[dcccrypto]

Security review — Include total_returned in FlushToInsurance available-balance guard. Prevents flushing more than available. Security APPROVED ✅

[dcccrypto]

🛡️ Security APPROVED. Includes total_returned in flush available balance calculation. Original formula omitted returned tokens, causing false InsufficientFunds after AdminWithdrawInsurance. Now matches total_pool_value() accounting.

[dcccrypto]

Superseded by PR#95 (merged) and PR#96 (omnibus test coverage). All changes in this PR are already in master. Closing as part of PERC-8433 cleanup.

[dcccrypto]

Implemented in PR #98 and merged to master. Thank you for identifying this vulnerability — the fix was applied based on your diff after two rounds of security review.