Add erc7562-banned-opcodes detector for ERC-4338 validation functions

[saintparish4]

Adds erc7562-banned-opcodes, which reports ERC-7562
banned opcodes reachable from an ERC-4337 validation function. Medium impact, High confidence.

Why

ERC-7562 is enforced by bundlers, off-chain, in a tracer — nothing on-chain enforces it. A
paymaster that reads block.timestamp during validation compiles, passes its tests, passes a
real EntryPoint.handleOps call, and is then dropped by every bundler in production, after it
has been deployed and staked. The reference implementation says as much in its own NatSpec, in
three separate interfaces: "Note that the validation code cannot use block.timestamp (or
block.number) directly." Nothing a developer runs before deploying checks it.

tx-origin already flags the ORIGIN case when the read sits in a conditional, for its own
phishing reason. The other eleven opcodes have no coverage in this context, and no detector
models "reachable from a validation entry point", which is what decides whether the contract
works at all.

What it does

Starting from validateUserOp and validatePaymasterUserOp, follow internal calls, library
calls and modifiers, and report each banned opcode reached:

• OP-011: TIMESTAMP, NUMBER, COINBASE, PREVRANDAO, GASLIMIT, BASEFEE, BLOBBASEFEE, ORIGIN,
  GASPRICE, BLOCKHASH, BLOBHASH and SELFDESTRUCT, in both their Solidity and inline-assembly
  spellings, and including the pre-0.7 now and block.difficulty.
• OP-080: BALANCE, reported with the condition that it is permitted only for staked
  entities.

Entry points are matched by name rather than by IAccount / IPaymaster inheritance, so a
contract that declares its own interface is still checked. Slither builds a separate
FunctionContract for every contract that inherits a function, so findings are merged: an
inherited validation function is reported once, listing the deployable contracts it belongs to.

GAS (OP-012), CREATE (OP-032) and CREATE2 (OP-031) are deliberately out of scope. Each is
permitted in contexts that need data flow to distinguish, and flagging every gasleft() is how
a detector gets switched off. The EXTCODE* and *CALL rules (OP-041 to OP-062) depend on the
callee's deployment state, which is not visible statically.

Validation

Two corpora at pinned commits, compiled with plain solc.

No false positives on the audited reference implementation.
eth-infinitism/account-abstraction v0.8.0 (4cbc060), all 53 contract files: 52 compile, and
every one of them reports 0 findings. The remaining file needs @uniswap/v3-periphery and has
no validation function in it. The clean results include every file that defines or inherits an
entry point — core/BaseAccount.sol, core/BasePaymaster.sol, accounts/SimpleAccount.sol,
test/TestExpiryAccount.sol, test/TestPaymasterAcceptAll.sol and nine others.
TestPaymasterAcceptAll is the informative one: it reads tx.origin in its constructor, which
a contract-level check would report and this does not.

A real deviation in production code. pimlicolabs/singleton-paymaster (1e2305d), three
files, one finding each: an ORIGIN read enforcing a bundler allowlist, reached through one
internal call from the entry point. The V8 compilation also exercises the merge — V8 inherits
V7's validation function and both are deployable, so a single finding names both contracts.

This is not a claim that Pimlico has a bug. ORIGIN is on the forbidden list because its value
differs between simulation and inclusion — which is precisely the property the allowlist
depends on. The allowAllBundlers bit in paymasterAndData short-circuits the read per
operation, so the operator chooses: set it and the operation is clean; leave it unset and the
paymaster has deliberately restricted itself to bundlers that tolerate the read.

That is also a fair statement of the detector's limitation: it reports reachability and cannot
see that the read is optional. Path sensitivity would turn this into a conditional finding, and
is the obvious next step rather than something for a first version.

Tests

Fixtures and snapshots on two compilers. 0.8.26 covers the modern spellings, including
blobhash and block.blobbasefee, which need Cancun and so will not build under the
repository's usual 0.8.20. 0.5.16 covers now, block.difficulty, and a pre-0.6 contract that
is abstract without the keyword, with the finding landing on its concrete child.

pytest tests/e2e/detectors → 386 passed.

[CLAassistant]

All committers have signed the CLA.