refactor(chain): commit block digest over a transaction root

[erenyegit]

What

Fold a block's transactions into a domain-separated transaction root and
commit the block digest over that root instead of inlining every transaction.
Add a compact, digest-authenticated BlockHeader that carries every
non-transaction digest field plus the transaction root, so a verifier can
reproduce and check a block's digest without the full block, then read the
authenticated state_root / state_range out of it.

This is the foundation the bridge's certified checkpoint needs: to authenticate
a foreign chain's state root from a finalization certificate (which signs a
block digest), the verifier must recompute that digest from a compact header
rather than the entire block.

Changes

• Transaction root computation — domain-separated sequential-hash commitment over
  the ordered transaction list.
• Block digest now commits over the transaction root (via a shared
  block_digest), not the inlined transactions.
• BlockHeader<Ext> — context, parent, height, timestamp, transaction_root,
  reshare_log, extension, state_root, state_range + codec.
• Block::header() and BlockHeader::digest() (reproduces the source block's
  digest exactly).

Consensus-breaking

This changes every block's digest and must land before testnet.

Note on naming

The transaction root is a sequential-hash commitment, not a Merkle tree: it
authenticates the exact ordered transaction list (count, order, and bytes all
matter) but does not support per-transaction inclusion proofs.

Tests (chain/tests/block_header.rs)

• block.header().digest() == block.digest()
• changing a transaction changes the root and digest
• reordering transactions changes the root and digest
• empty vs non-empty transaction lists differ
• changing state_root / state_range changes the header digest
• header codec round-trips

Follow-up

The bridge certified-checkpoint PR builds on this: the consensus extension will
verify a foreign finalization certificate against a BlockHeader and store
checkpoint_id -> state_root + state_range.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
chain/src/block.rs	92.85%	0 Missing and 9 partials ⚠️

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