fix: guard CalcBaseFee against divide-by-zero from malformed extraData

[latifkasuli]

Summary

Fixes #757

CalcBaseFee unconditionally decoded Holocene/Jovian extraData from the parent header and passed the result to calcBaseFeeInner. The decoders use best-effort logic and return 0, 0 on malformed input, which causes a divide-by-zero panic:

• elasticity == 0 panics on integer division (parent.GasLimit / elasticity)
• denominator == 0 panics on big.Int.Div

Root cause

The precondition documented in CalcBaseFee ("It is assumed the parent Header has valid extraData") is not upheld by all callers. Many non-consensus callers (RPC pending tx, fee history, GraphQL, txpool repricing, blobpool, miner, simulate, t8ntool) use CalcBaseFee on headers they assume are valid. A malformed parent header already present locally will trigger the panic.

Additionally, chains activating Holocene or Jovian at genesis can hit this because:

• Genesis ExtraData is written without Optimism-specific validation (core/genesis.go)
• Consensus header validation explicitly skips the genesis block (consensus/beacon/consensus.go)
• Block 1 calls CalcBaseFee on the genesis parent, triggering the panic

Changes

1. consensus/misc/eip1559/eip1559.go: Gate the DecodeOptimismExtraData call behind ValidateOptimismExtraData. If the parent extraData is invalid, chain-config defaults are preserved instead of zero values. No API signature changes.

2. core/genesis.go: Reject invalid Optimism extraData at genesis commit time for chains that activate Holocene or later at genesis.

3. consensus/misc/eip1559/eip1559_test.go: Add regression tests:

   • TestCalcBaseFeeOptimismMalformedExtraData: 10 sub-tests covering empty, short, wrong-version, wrong-length, zero-denominator, zero-elasticity, and both-zero extraData for both Holocene and Jovian parent times. Asserts no panic.
   • TestCalcBaseFeeOptimismMalformedFallsBackToConfig: Verifies that malformed extraData produces the same base fee as chain-config defaults (Canyon denominator/elasticity), not zeros.

What this does NOT change

• ValidateHolocene1559Params is not tightened. (0, 0) in payload attributes is an intentional sentinel per the Holocene spec.
• Consensus header validation already rejects zero denominator/elasticity in extraData via ValidateOptimismExtraData in beacon.verifyHeader.

Test plan

• All 30 existing + new tests pass (go test ./consensus/misc/eip1559/ -v)
• go build ./core/ compiles cleanly
• CI passes