Optimize transaction processing via encoded-tx trie root and fast block re-encode

[Copilot]

Changes

Avoid redundant RLP encoding in transaction processing hot paths:

• Encoded transaction trie root: TxTrie.CalculateRoot(ReadOnlySpan<byte[]>) computes trie root directly from RLP-encoded transactions. ExecutionPayload.TryGetBlock uses this to avoid re-encoding transactions already provided by CL.

• Fast block re-encode: Block.EncodedTransactions caches pre-encoded transaction bytes in SkipTypedWrapping format. BlockDecoder.Encode reuses these when available, avoiding re-encoding during block serialization. Handles legacy/typed transaction format differences correctly.

• Trie parallelization: Branch node RLP encoding now parallelizes only when ≥4 non-null children exist, reducing overhead on sparse branches.

Types of changes

• Optimization
• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

Tests verify encoded and decoded paths produce identical trie roots and block RLP.

Documentation

Requires documentation update

• Yes
• No

Requires explanation in Release Notes

• Yes
• No

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