Fix validation blocking async tasks & upgrade mega-evm dependency

[flyq]

Summary

This PR improves the stateless validator's performance by introducing a thread pool for CPU-intensive validation tasks and upgrades the mega-evm dependency with refactored hardfork implementations.

Purpose

To address two issues encountered in testnet v2:

1. The asynchronous runtime was blocked by validate_one, resulting in thousands of blocks being validated without the reporter providing any updates. At the time, the server was running both the witness generator and the validator simultaneously; the generator was using 200% CPU, and the validator, with 32 workers, was using 800% CPU.
2. The issue of the release build size being excessively large. Before the fix, the validator size was 222MB; after the fix, it was 8MB.

Changes

1. Use thread pool for validation to avoid blocking async tasks

• Wrapped validate_block() call in task::spawn_blocking() to run CPU-intensive validation in a dedicated thread pool
• This prevents blocking of other async tasks (reporter, tracker, etc.) during validation
• Adjusted function signatures to properly handle Arc<ChainSpec> for thread-safe sharing

2. Optimize release build profile

• Changed to lto = "fat" for better link-time optimization
• Added strip = true to reduce binary size
• Added panic = "abort" for smaller binary and faster panic handling
• Removed incremental compilation for release builds

3. Upgrade mega-evm & refactor chainspec hardfork implementation

• Updated mega-evm to commit 3b28c205c8c2f89730a4dd88abe47e1831c6fac9
• Replaced local MegaethHardfork and MegaethHardforks definitions with MegaHardfork and MegaHardforks from mega-evm
• Simplified chain_spec.rs by removing 43 lines of redundant hardfork code
• Implemented MegaHardforks trait for ChainSpec

Files Changed

• Cargo.toml - Updated dependencies and release profile
• Cargo.lock - Updated lockfile
• bin/stateless-validator/src/main.rs - Added spawn_blocking for validation
• validator-core/src/chain_spec.rs - Refactored hardfork implementation