fix: use senior sub-pool formula for withdrawal when tranches are enabled

[0x-SquidSol]

Summary

When tranches are enabled, process_withdraw used the global pool formula for senior LP withdrawals:

// WRONG when tranches enabled
collateral = lp_amount * total_pool_value / total_lp_supply

The correct formula uses the senior sub-pool only:

// CORRECT
collateral = lp_amount * senior_balance / senior_lp_supply
// where:
//   senior_balance   = total_pool_value - junior_balance
//   senior_lp_supply = total_lp_supply  - junior_total_lp

Why This Is a Security Issue

Using the global formula when tranches are active mixes junior-backed collateral into the senior valuation:

1. Fee redistribution attack: If junior_fee_mult_bps > 10000, fees are redistributed so junior balance grows faster per LP token than senior. The global formula over-values senior LP (includes junior's above-par collateral), letting senior holders drain funds belonging to junior holders.

2. Loss absorption mismatch: If junior absorbs losses (junior_balance < original), the global formula under-charges senior holders for the loss. Senior withdrawals appear entitled to more than their actual share of remaining vault.

Note: junior withdrawals already correctly used calc_junior_collateral_for_withdraw(junior_lp, junior_bal, lp_amount). This fix applies the same pattern to senior: calc_senior_collateral_for_withdraw(senior_lp, senior_bal, lp_amount). When tranches are disabled, behaviour is unchanged (falls through to global formula).

Test plan

• All existing tests pass (cargo test)
• Verify senior withdrawal with active tranches uses senior sub-pool value
• Verify withdrawal with tranches disabled still uses global formula

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ℹ️ Review info

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Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 8ca3440a-b94e-4496-b960-2fd1280fac29

📥 Commits

Reviewing files that changed from the base of the PR and between 1881738 and dd80047.

📒 Files selected for processing (1)

• src/processor.rs

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[dcccrypto]

Security review — Senior sub-pool formula for withdrawal when tranches enabled. Correct accounting prevents senior depositors from claiming junior funds. Security APPROVED ✅

[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Superseded by PR#95 (merged) and PR#96 (omnibus test coverage).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.