Enable ethrex's rayon feature on the embedded node

[0w3n-d]

Issue: none (one-line dependency fix; happy to file a bug issue if you want it tracked)

What this PR does

Adds rayon to the ethrex-blockchain, ethrex-common and ethrex-vm pins.
It is a default feature of all three; our pins set default-features = false,
and only the ethrex cmd crate — deliberately not a dependency — adds it back,
so the embedded node has never had it.

Without it the node cannot follow the head on an Amsterdam chain. ethrex gates
the parallel BAL execution path on rayon
(crates/vm/backends/levm/mod.rs:485) but not the caller that decides whether
to create the merkleizer channel (crates/blockchain/blockchain.rs:1026). With
a BAL supplied and both bal_parallel_* options at their defaults, the caller
skips the channel, LEVM drops the BAL, and the sequential path errors with
sequential execution path called without a merkleizer Sender. Every
newPayload that carries a BAL fails. P2P sync is unaffected, because it
passes no BAL — which is why a simulator syncs to the head and then stalls
one block later. Observed on plataberget at block 176481.

The feature also gates the mempool prewarm (Mempool prewarm requires the rayon feature; disabled) and the block warmer (warmer: 0.00 ms), both now
active.

What this PR deliberately does not do

• No change to the pinned ethrex rev, and no patch to ethrex. The
  cfg-gating inconsistency is an upstream bug; this PR only stops us from
  triggering it. An upstream report is worth filing separately.
• No change to ethrex-p2p, ethrex-levm or ethrex-storage pins.
  ethrex-levm picks up rayon transitively through ethrex-vm, and
  ethrex-p2p uses rayon unconditionally.
• Does not touch the aws-lc-rs/cc constraint that the comment above these
  pins documents. rayon is unrelated to it.

Tests

No new tests. Nothing in the suite executes an Amsterdam block with a BAL
through add_block, so no unit test can observe this; the failure is a
feature-resolution problem, not a logic one.

Verified two ways:

• cargo metadata before and after. Before, all four ethrex crates resolved
  without rayon; after, all four have it. Identical under --all-features.
• Existing checks: just fmt-check, cargo clippy --all-features --no-deps -- -D warnings, and just test (414 passed, 0 failed, 8 ignored).

Still needs a redeploy on plataberget to confirm the two log lines are gone.
Note that this means the simulation role has never validated an Amsterdam block
through the engine path, so earlier sim results on this stack are not
trustworthy.

Reviewer checklist

• CI (lint, unit-test) is green
• Matches the linked issue/step
• No unexplained scope creep or unrelated files touched

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