feat(params,genesis): schedule devnet gas2500x fork

[gzliudan]

Summary

Schedule the devnet gas2500x hard fork at block 1,385,000, estimated to activate on 2026-09-08 10:02 CST (02:02 UTC) The 2500x fork is enabled only on devnet; testnet and mainnet keep Gas2500xBlock nil.

Motivation & Context

The devnet needs a coordinated gas2500x activation so all validators switch to the new gas-tier rules at the same block. The activation block is derived from the live devnet stats snapshot: head 1,185,928 observed at 2026-09-03 19:26:54 CST with a measured 2.0s block time, targeting 2026-09-08 10:00 CST.

Changes

• params/config_networks.go: add Gas2500xBlock: 1385000 to DevnetChainConfig, placed after Gas50xBlock(0) and before EIP1559Block(25000) so it satisfies CheckConfigForkOrder's BASEFEE window rule.
• genesis/devnet.json: add "gas2500xBlock": 1385000 so XDC init --devnet and XDC --devnet schedule the identical fork (covered by TestEmbeddedGenesisConfigMatchesParams).
• params/config_networks_test.go: remove devnet from TestDefaultXDCNetworksDoNotEnableGas2500xFork; testnet/mainnet remain asserted to nil.

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