Cyfrin Savings Fix — audit remediation (L-1, L-2, I-3, I-4)

[FabienCoutant]

Remediation of the Cyfrin "Parallel Savings Fix" report (v1.0, 2026-06-04), stacked on top of the audited branch (#27). One commit per finding.

Code findings

• [L-1] initializeStoredAssets() now reverts StaleAccrual() when block.timestamp - lastUpdate > 30 min. With a fresh accrual there is no unaccrued-interest delta to compound against an inflated storedAssets. Pre-upgrade step: call setRate to refresh accrual right before the upgrade, otherwise the upgrade tx itself reverts.
• [L-2] unpause() advances lastUpdate to the unpause timestamp instead of _accrue()-ing the paused span, so the paused interval is dropped rather than minted. Pausing now halts emission (pause() still settles yield up to the pause moment).
• [I-3] initializeStoredAssets() gains the restricted modifier — only governance can seed storedAssets, independent of whether the upgrade runs atomically.
• [I-4] initialize() sets lastUpdate = block.timestamp, bounding the first accrual window to "since deployment" rather than "since epoch". Defense-in-depth for future fresh deployments (inert for the existing proxy, since initialize is not re-run on upgrade).

Operational (no code change)

• [I-1] Decommission order: call setRate(0) while the vault is still a minter (final _accrue succeeds), then revoke the minter role. Reverse order traps the rate.
• [I-2] Never set Savings as a surplus payee: surplus is paid by direct transfer, which the donation guard excludes from totalAssets() and routes to recoverSurplus — it never reaches savers.

Tests

MAX_STORED_ASSETS_INIT_STALENESS (30 min) and StaleAccrual() added. New coverage for access control + staleness on initializeStoredAssets; pause/unpause and lastUpdate assertions updated for the new behavior. Local CI green: solhint, forge build --sizes, test:unit (222/222).