Add ERC4626 spec compliance test suite

[thedavidmeister]

ReceiptVault / ERC20PriceOracleReceiptVault / OffchainAssetReceiptVault generalize ERC4626 with per-id receipts, but there's no test suite that pins the spec clauses themselves. Each test file targets a single function's behaviour; nothing exercises the cross-function invariants the spec demands.

Concrete evidence the gap matters: #302's testMaxFunctionsDoNotRevertOnOracleRevert (added to assert oracle errors don't reach the MUST-NOT-revert surface) flagged maxWithdraw(owner, 0) panicking with Panic(0x12) from divide-by-zero inside _calculateRedeem. That's a direct violation of ERC4626's "MUST NOT revert" clause for max*, oracle-independent, and unaddressed today. The PR test was scoped narrowly to the oracle hypothesis (id bounded > 0) to avoid expanding scope, but the underlying defect persists.

Suggested coverage:

• max* MUST NOT revert for any input (including id == 0, zero balances, paused state, broken oracle).
• preview* matches the corresponding mutating call when called from the same caller with the same args under the same state.
• convertToShares / convertToAssets round-trip behaviour and inverse relationship.
• totalAssets consistency under deposit / withdraw / mint / redeem.
• asset() immutability post-init.
• Decimals offset / rounding direction per ERC4626 spec table.

Each clause should apply to both ERC20PriceOracleReceiptVault and OffchainAssetReceiptVault, with the receipt-id dimension fuzzed.