feat: scaffold in-process ethrex execution-engine crate (phase 0)

First step toward integrating ethrex in-process as a library crate (counterpart
to #367, which integrates ethrex out-of-process over the Engine API). This is a
de-risk spike proving ethrex embeds as an unmodified pinned git dependency and
compiles cleanly inside the workspace before any functional code is written.

- New crate crates/net/ethrex-engine (ethlambda-ethrex-engine) linking the three
  core ethrex library crates (ethrex-common, ethrex-storage, ethrex-blockchain),
  pinned to rev de9b249b. No functionality yet; ExecutionEngine impl follows.
- ethrex ships its own EVM (levm) and devp2p, so there is no revm/libp2p/
  ethereum_ssz clash; lockfile resolves with zero unification conflicts. The
  heavyweight ethrex-rpc crate is intentionally not a dependency.
- docs/plans/ethrex-inprocess-poc.md documents the design and phased plan.

Author: pablodeymo