feat(fee): chainspec floor with code-driven activation schedule

[AshinGau]

Summary

Replaces the global hardcoded 50 Gwei floor from #335 with a chainspec-field + code-driven schedule. Non-Gravity chainspecs (Ethereum mainnet history sync) keep upstream EIP-1559 semantics; Gravity chainspecs apply the floor according to a per-branch schedule.

Design

Genesis (config.gravityMinBaseFee): carries only the floor value applicable to the latest schedule segment. Its presence marks the chainspec as Gravity; absence means no floor.

Code (this branch — main): crates/chainspec/src/constants.rs::GRAVITY_MIN_BASE_FEE_ACTIVATION_BLOCK = 0, with a single-segment schedule [0, ∞) returning the genesis value.

Trait helper: EthChainSpec::gravity_min_base_fee_at_block(block) -> Option<u64> — defaults to None, ChainSpec impl encodes the schedule. Used by both next_block_base_fee (consensus / pipe-exec block production) and EthereumPoolBuilder::build_pool (pool admission floor, queried at head + 1 and combined with the operator CLI override via max()).

Compatibility

Scenario	Behavior
Ethereum mainnet history sync	Genesis lacks gravityMinBaseFee → schedule returns None for any block → upstream EIP-1559, no clamp, pool admission floor = MIN_PROTOCOL_BASE_FEE (7 wei). Fixes #335 breaking the Ethereum mainnet London transition (block 12,965,000) where actual base_fee = 1 Gwei.
Gravity main	Genesis sets gravityMinBaseFee = 50_000_000_000, activation block = 0 → floor enforced from genesis. Pipe-exec and pool admission both clamp at 50 Gwei.
Released testnet branch	Cherry-pick this commit and override GRAVITY_MIN_BASE_FEE_ACTIVATION_BLOCK with the rolling-upgrade activation height N. Before block N the schedule returns None (matches pre-floor behavior, rolling-upgrade safe); from block N onward it returns the genesis value.

Future upgrade method

When a future release changes the floor from value X to Y at block N:

1. Update genesis JSON: gravityMinBaseFee = Y
2. Extend the schedule helper with a new historical segment:

fn gravity_min_base_fee_at_block(&self, block: u64) -> Option<u64> {
    if block >= GRAVITY_MIN_BASE_FEE_ACT_BLOCK_N {
        self.gravity_min_base_fee   // genesis value (Y)
    } else if block >= GRAVITY_MIN_BASE_FEE_ACT_BLOCK_M {
        Some(50_000_000_000)        // hardcoded historical X
    } else {
        None
    }
}

Historical floor values are hardcoded in code, so a node restarted across multiple hardforks always validates older blocks correctly from the binary's full schedule. Genesis-only designs (which carry only the current state) cannot guarantee this.

Drive-by

OpChainSpec was missing the gravity_hardforks impl introduced in #309 — a pre-existing bug that blocked workspace compile. Added forwarding impl for gravity_hardforks and gravity_min_base_fee_at_block.