feat(node): multi-blob derivation support (V2 batch)

[curryxbo]

Summary

Complete the last remaining piece on feat/multi_batch: node-side derivation support for V2 multi-blob batches. V2 support on the contracts / prover / challenger sides has already landed on feat/multi_batch; this PR fills in the derivation gap so that multi-blob batches can be decoded end-to-end.

Key design points (aligned with morph-specs spec-002 §3.2 step 7 and §4.2.5):

• Concat first, decompress once. tx-submitter for V2 compresses the entire batchBytes as a single zstd stream and then splits it across blobs by MaxBlobBytesSize. Only blob[0] carries a valid zstd frame header; blob[1..N-1] are middle bytes. The previous implementation decompressed each blob independently and appended the results, which fails as soon as more than one blob is used. The new path iterates blobs in tx.BlobHashes() order, extracts raw scalar bytes, concatenates into a single buffer, and calls zstd.DecompressBatchBytes exactly once.

• Strict reordering against tx.BlobHashes(). The Beacon API does not guarantee sidecar order matches the L1 tx's declared blob hashes. Any reordering would corrupt the concatenated zstd stream. We index sidecars by the KZG-derived versioned hash and reassemble the local BlobTxSidecar in exactly the order declared by the tx.

• Per-blob VerifyBlobProof. KZG proof verification runs per blob before concatenation. A corrupt blob is located directly instead of being misattributed to a decompression failure downstream.

• V2 header parsing. V2 reuses V1's 257-byte wire layout, but the semantics of offset 57 change from "single versioned hash" to "aggregated hash keccak(hash[0]||...||hash[N-1])".

  • BlobVersionedHash() returns an error on V2+ to prevent callers from silently treating the aggregated hash as a single blob hash.
  • New BlobHashesHash() accessor returns the aggregated hash; it errors on V0/V1.

Changes

node/types/batch_header.go

• Add BatchHeaderVersion2 = 2 and expectedLengthV2 = 257.
• validate() accepts the V2 length.
• Split BlobVersionedHash() (V0/V1 only) and BlobHashesHash() (V2+ only), each guarded by a version check.

node/derivation/batch_info.go

• Rewrite ParseBatch() to concatenate all blob raw bytes in tx order first, then perform a single DecompressBatchBytes.

node/derivation/derivation.go

• fetchRollupDataByTxHash() reorders beacon sidecars by tx.BlobHashes() via a KZG-derived hash index.
• Enable per-blob VerifyBlobProof.

Tests

node/types:
  TestBatchHeader                                    PASS
  TestBatchHeaderV2                                  PASS
  TestBlobHashesHashUnavailableForLegacy             PASS

node/derivation:
  TestParseBatchSingleBlob                           PASS
  TestParseBatchMultiBlob                            PASS  (random incompressible payload forces compressed size > MaxBlobBytesSize, guaranteeing split across 2 blobs)
  TestParseBatchMultiBlobConcatDecompressInvariant   PASS

Test Plan

• go build ./... passes.
• Unit tests: single-blob regression + multi-blob cross-boundary parsing + V2 header parsing + V0/V1 guards.
• (integration) Devnet end-to-end V2 multi-blob run: tx-submitter emits a multi-blob batch → rollup commitBatch → node derivation → state replay.
• (integration) V1 parent → V2 child transition batch: verify blockCount derivation is correct across the version bump.

Related

• Spec: morph-l2/morph-specs/specs/2026/Q2/spec-002-multi-blob-batch-submission
• Base branch commits: V2 contracts (d78251c4 / 56bd83ff / 9ae6e516), prover multi-blob (2711f073 / 9894b040 / edf823ff), challenger (8c419a78).

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