refactor(l1): remove unneeded clone and Arc

[MegaRedHand]

Motivation

We had a useless Arc in the layering module.

Description

This PR removes the Arc, which also lets us remove a clone we had in the commit function.

[Copilot]

Pull Request Overview

This PR removes redundant Arc wrapping from the nodes field in TrieLayer, simplifying the data structure since TrieLayer instances are already wrapped in Arc at the cache level.

• Removed nested Arc wrapper from TrieLayer::nodes field
• Updated constructor to directly assign the HashMap without wrapping
• Simplified iterator chain to use the HashMap directly

Comments suppressed due to low confidence (1)

crates/storage/trie_db/layering.rs:181

• With the removal of Arc from the nodes field, cloning TrieLayer at line 180 now performs a deep clone of the entire FxHashMap, which could be expensive for large maps. Consider if this fallback case (when Arc::try_unwrap fails) is frequent enough to warrant keeping the Arc<FxHashMap> for cheaper clones, or document this trade-off.

        let layer = match Arc::try_unwrap(self.layers.remove(&state_root)?) {
            Ok(layer) => layer,
            Err(layer) => TrieLayer::clone(&layer),
        };

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