chore: add network upgrade skeleton for FIP-0115 activation epoch

[coderabbitai]

Summary

A network upgrade skeleton is needed to wire the FIP-0115 activation epoch into the chain/network configuration, so that the premium-based base-fee branch introduced in #6702 can activate at runtime.

Currently, PLACEHOLDER_NEXT_UPGRADE_HEIGHT in src/chain/store/base_fee.rs is set to ChainEpoch::MAX, which effectively disables the FIP-0115 execution path. Once the actual upgrade height is determined, it should be sourced from the network configuration (e.g., ChainConfig) rather than a compile-time sentinel.

Tasks

• Add a new network upgrade height entry to ChainConfig (or equivalent) for the FIP-0115 activation epoch
• Replace PLACEHOLDER_NEXT_UPGRADE_HEIGHT in src/chain/store/base_fee.rs with the configured value
• Update all callers that import PLACEHOLDER_NEXT_UPGRADE_HEIGHT to read the configured activation epoch
• Adjust tests/fixtures to inject the desired activation epoch for deterministic behavior

References

• PR: feat: impl basefee change from FIP-0115 #6702
• Review comment: feat: impl basefee change from FIP-0115 #6702 (comment)
• Requested by: @LesnyRumcajs