ci: add gravity genesis fixture regeneration

[ByteYue]

Summary

• add a reth-side Gravity hardfork genesis fixture source of truth under crates/pipe-exec-layer-ext-v2/execute/fixtures
• add regen.sh to regenerate gravity_hardfork.json from the pinned gravity_chain_core_contracts ref using the standalone contracts genesis-tool path
• wire the gravity pipe CI job to restore/cache the generated fixture and diff it against the committed baseline
• update hardfork testing docs from the old gravityHardforks wording to the current top-level hardfork fields

Addresses Galxe/gravity-audit#672.

Validation

• bash -n crates/pipe-exec-layer-ext-v2/execute/fixtures/regen.sh
• crates/pipe-exec-layer-ext-v2/execute/fixtures/regen.sh --print-metadata
• python3 -m json.tool crates/pipe-exec-layer-ext-v2/execute/fixtures/validator_genesis.json
• validator fixture compared equal to gravity-testnet-v1.0.0:genesis-tool/config/genesis_config_single.json plus chainId = 7771625
• git diff --check
• cargo check -p reth-pipe-exec-layer-ext-v2 --tests --locked

Full local regeneration was not run because this machine does not have forge; CI installs Foundry for cache misses.