fix(perps): enforce native equity health on withdrawals

[rndrntwrk]

Summary

Fixes the native perps health-check issue found during the contract audit.

Before this patch, AgentPerpEngineNative checked leverage using raw margin only. After an adverse oracle move, a position could have negative equity while still passing the raw notional / margin check, and withdrawMargin() did not sync the oracle before evaluating the withdrawal.

This patch brings the native health gate in line with the ERC20 engine: health is evaluated against equity, where equity = margin + unrealizedPnl.

Security Impact

• Native perps health checks now reject equity <= 0.
• Max leverage is computed as notional / equity, not notional / raw margin.
• withdrawMargin() now requires a live market, syncs the oracle, settles funding, subtracts the withdrawal, then checks equity before sending ETH.
• modifyPosition() uses the same equity-aware health gate after position changes.

Verification

• Red-path regression was confirmed before the fix: testNativeWithdrawMarginRejectsNegativeEquityAfterOracleMove failed because the withdrawal did not revert.
• env FOUNDRY_LIBS='["../../node_modules","lib"]' forge test --match-path test/perps/AgentPerpEngine.t.sol --match-test testNativeWithdrawMarginRejectsNegativeEquityAfterOracleMove -> 1 passed, 0 failed
• env FOUNDRY_LIBS='["../../node_modules","lib"]' forge test --match-path test/perps/AgentPerpEngine.t.sol -> 49 passed, 0 failed
• env FOUNDRY_LIBS='["../../node_modules","lib"]' forge test --match-path test/perps/AgentPerpEngineFuzz.t.sol -> 2 passed, 0 failed
• env FOUNDRY_LIBS='["../../node_modules","lib"]' forge test --match-path test/adversarial/PmPerpsAdversarial.t.sol -> 11 passed, 0 failed
• git diff --check -> pass

Staging Policy

This PR targets main and is intentionally draft while review is pending. The same patch will be replayed into enoomian/staging for staging deployment and testing, but this effective PR remains open as the merge vehicle.

Follow-up

This PR fixes the withdrawal/health-gate portion. The separate native liquidation accounting finding remains open because it requires PnL realization, insurance draw, and bad-debt accounting changes in the liquidation path.