docs: document mini-block sequencer signatures

[Troublor]

Summary

Documents mini-block sequencer signatures, which back the preconfirmation guarantee the docs previously asserted without explanation:

• docs/mini-block.md: new "Sequencer Signatures" section — the sequencer signs every mini-block header (keccak256(rlp(header)), raw secp256k1 ECDSA, no EIP-191 prefix, ecrecover-compatible), the key is registered and rotatable on-chain via the SequencerRegistry system contract (0x6342...0006, since Rex5). Includes the 8-field RLP layout table and a "Verifying a Mini-Block Signature" guide with a TypeScript (viem) example, plus rotation guidance (sequencerAt vs currentSequencer) and a finality caveat. Also fixes the stale "no Merkle roots" header description (mini-block headers carry transaction/receipt roots).
• docs/dev/read/rpc/eth_subscribe.md: adds the three miniBlocks payload fields the node serializes but the table omitted: transaction_root, receipt_root, signature (r/s/yParity object).
• docs/dev/read/realtime-api.md and docs/dev/faq.md: one-line tie-ins linking the preconfirmation claims to the new section.

Test plan

• TypeScript example validated end-to-end against live endpoints: captured miniBlocks payloads on Mainnet and Testnet, recomputed the header hash, recovered the signer, and matched SequencerRegistry.currentSequencer() / sequencerAt(blockNumber) on both networks.
• Facts cross-checked against mega-reth origin/develop (crates/megaeth/realtime/src/mini_block/mini_block.rs, crates/megaeth/utils/src/crypto.rs, crates/megaeth/rpc/src/pubsub.rs) and the SequencerRegistry spec in mega-evm.
• prettier run on all touched files.

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This PR was generated by an automated agent.

[github-actions]

Good PR overall — the new section is accurate, well-structured, and the code examples are clearly validated against live endpoints. Two convention issues to fix before merging:

1. Heading case (×2): ## Sequencer Signatures and ### Verifying a Mini-Block Signature should be sentence case per the docs-wide rule. See inline comments.
2. Terminology (×7): on-chain / off-chain should be onchain / offchain throughout — this is explicitly listed in the terminology table in docs/AGENTS.md. Flagged on the first occurrence with a full list of lines.

[github-actions]

Review

Previous threads: All three prior comments (heading sentence case ×2, on-chain → onchain ×7) have been addressed in the latest push. (Thread resolution isn't available to this integration, but they can be resolved manually.)

New content looks good overall: the Sequencer Signatures section is accurate, well-structured, and follows the narrative tone expected of a top-level page. The RLP field table, TypeScript/Solidity examples, and sequencerAt vs currentSequencer guidance are all solid.

One remaining issue: link text in two files uses Sequencer Signatures (title case) while the target heading is ## Sequencer signatures (sentence case). Inline suggestions posted.

[github-actions]

All five previous threads are addressed in the latest push:

• Both heading-case threads (## Sequencer signatures, ### Verifying a mini-block signature) — sentence case ✓
• The on-chain → onchain thread — verified clean across the full file (no remaining hyphenated instances) ✓
• Both link-text case threads in realtime-api.md and eth_subscribe.md ✓

(Unable to resolve threads programmatically due to token permissions — please resolve manually.)

The new content looks correct:

• RLP field order and types match the spec reference.
• TypeScript int() helper correctly handles the zero-as-empty-bytestring RLP edge case.
• Solidity ecrecover uses yParity + 27 correctly; signer != address(0) guard is present.
• sequencerAt(blockNumber) used for historical verification, currentSequencer() for live — guidance is accurate.
• All GitBook blocks ({% hint %}, {% tabs %}) are properly closed.
• Frontmatter, relative links, and SUMMARY.md are unaffected (no new pages added).

LGTM.

[github-actions]

All five prior review threads have been addressed:

• Heading case fixed: ## Sequencer signatures, ### Verifying a mini-block signature
• All on-chain / off-chain instances replaced with onchain / offchain throughout
• Link text casing in realtime-api.md and eth_subscribe.md corrected to sentence case

The new content looks good:

• Technical accuracy: the 8-field RLP layout, signing algorithm (keccak256(rlp(header)), raw secp256k1, no EIP-191), and sequencerAt vs currentSequencer guidance all match what was cross-checked against the source.
• TypeScript example is sound — the int() helper correctly produces minimal big-endian byte strings for RLP (zero → empty bytes 0x80), hash fields are passed through directly, and recoverAddress applies no EIP-191 prefix.
• Pre-Rex5 caveat is handled with a hint block adjacent to the code, not buried in a footnote.
• Header-size table fix ("no state root" instead of "no Merkle roots") is accurate.
• Relative link paths are correct from all four changed files.

LGTM.