perf(trie): use rayon spawn_broadcast for proof workers

[gakonst]

Summary

Replace the for loop + pool.spawn() pattern with pool.spawn_broadcast() for spawning proof workers.

Changes

• Use spawn_broadcast on storage and account proof worker pools in proof_task.rs, getting the worker ID from BroadcastContext::index()
• Add Sync bound to Factory generic on ProofWorkerHandle::new (required by spawn_broadcast)
• Propagate the Sync bound to callers in mod.rs and multiproof.rs

Motivation

spawn_broadcast guarantees exactly one task per pool thread and provides a stable per-thread index via the broadcast context, eliminating the manual loop and per-iteration clones.

Prompted by: DaniPopes

[github-actions]

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Preview

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reth-engine-tree: patch
reth-trie-parallel: minor
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Switched proof workers from `spawn` to `spawn_broadcast` for improved parallelism and added `Sync` bound to database provider factory traits.

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