Account for Vow surplus in End.thaw debt calculation

[daatsuka]

This change touches End.thaw() in src/end.sol. The previous implementation enforced a hard require that vat.dai(address(vow)) must be exactly zero before thaw() could proceed, and then computed debt purely as vat.debt() − cure.tell(). That two-step approach created a precondition that could block emergency shutdown in practice — any residual DAI surplus sitting in the Vow would cause thaw() to revert, even though that surplus is a legitimate part of the system's outstanding obligations and should simply be subtracted from the total debt figure rather than treated as a blocker.

The updated logic removes the surplus-must-be-zero guard and instead folds vat.dai(address(vow)) directly into the debt calculation: debt = sub(vat.debt(), add(vat.dai(address(vow)), cure.tell())). This means thaw() now accounts for both the Cure-reported bad debt and the Vow's DAI balance in a single arithmetic expression, producing a net debt figure that accurately reflects what collateral holders are owed during shutdown. The design keeps the same sub / add safe-math helpers already used throughout end.sol, so there is no new overflow surface, and the change is intentionally minimal — one require removed, one line rewritten — to keep the audit footprint small.

The rationale is straightforward: surplus DAI held by the Vow is not external to the debt accounting; ignoring it (or refusing to proceed when it exists) leads to an unnecessarily strict shutdown path that the original Immunefi report correctly identified as problematic. I verified locally by running the full Foundry test suite (forge test) against the modified contract — all existing End tests pass without modification, and thaw() no longer reverts when the Vow carries a non-zero DAI balance.

Closes #373

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