feat(l1): charge EIP-8141 arbitrary frame signatures their own gas

[AnkushinDaniil]

Problem

The EIP-8141 signature scheme table assigns a verification gas cost to every scheme, including ARBITRARY:

scheme byte	scheme	verification gas
0x0	ARBITRARY	100
0x1	SECP256K1	2800
0x2	P256	6700

FrameTransaction::signature_verification_cost charged 0 for ARBITRARY entries (it was written against an older spec snapshot that predates the 100-gas entry). That under-charges total_gas_limit by 100 per ARBITRARY signature and also under-counts the validation-prefix verify-gas budget, so the MAX_VERIFY_GAS boundary sits 100 gas away from where a conformant client puts it.

Change

• Charge 100 gas per ARBITRARY signature in signature_verification_cost.
• Tests: total_gas_limit differs by exactly 2800 - 100 between an otherwise-identical SECP256K1 and ARBITRARY transaction (with an RLP-shape guard so the comparison isolates verification cost), and the prefix verify-gas budget flips from valid to VerifyGasBudgetExceeded exactly at the 100-gas boundary.

Evidence

Commands run on this branch:

cargo check -p ethrex-common                                # pass
cargo check -p ethrex-levm                                  # pass
cargo test -p ethrex-common transaction::tests
# 69 passed; 0 failed; 74 filtered out
cargo test -p ethrex-test --test ethrex_tests levm::eip8141_tests
# 75 passed; 0 failed; 751 filtered out
cargo fmt --all -- --check                                  # pass
git diff --check                                            # pass

Stack

Targets frames-devnet-0. Part of a series of independent EIP-8141 fixes, one PR per logical change, each standing alone against the same base. Siblings: #7040 (canonical low-s), #7043 (recovery id at ecrecover boundary), #7044 (intrinsic gas over payload bytes), #7046 (frame gas refunds), #7047 (frame receipt storage encoding), #7048 (skipped-frame status 2), #7049 (EIP-7623 calldata floor).

Consensus risk / limits

Consensus-affecting: it raises total_gas_limit (and the verify-gas budget accounting) for any frame transaction carrying ARBITRARY signatures. Covered by unit tests only; no cross-client conformance fixture yet.

[edg-l]

Superseded by #7004: signature_verification_cost on main already charges FRAME_SIG_SCHEME_ARBITRARY => 100, and main covers the scheme gas table in its own tests. Closing as fixed; thanks.