fix(pipe-exec): harden system transaction execution

[Lchangliang]

Summary

• gate DKG transcript size with alphaTime and keep legacy behavior before Alpha
• keep user transaction execution independent from Rust-level system transaction errors after Alpha
• preserve completed system transaction prefix when DKG triggers epoch change
• include execution epoch in results for SDK commit vote handling
• enforce emitter allowlists when extracting Gravity events from system receipts

Tests

• cargo +nightly fmt --all
• cargo test -p reth-pipe-exec-layer-ext-v2 --lib
• cargo check -p reth-pipe-exec-layer-ext-v2

Audit coverage

• https://github.com/Galxe/gravity-audit/issues/627
• https://github.com/Galxe/gravity-audit/issues/619
• https://github.com/Galxe/gravity-audit/issues/681
• https://github.com/Galxe/gravity-audit/issues/693