SDM: ecosystem integration - non-open source partners

## Context

Sequencer Defined Metering (SDM) introduces a synthetic trailing transaction of type `0x7d` (`PostExec`). It is exposed through standard Ethereum RPC rather than filtered out.

Before SDM activation, explorers, indexers, RPC libraries, tracing tools, and analytics platforms must be able to process blocks containing `0x7d` without rejecting the transaction or failing the entire block.

SDM also changes the gas-accounting surface for refunded user transactions:

- receipt `gasUsed` reflects canonical post-refund gas usage;
- `opGasRefund` exposes the gas refunded by SDM;
- tooling that needs pre-refund execution gas should use `gasUsed + opGasRefund`.

The synthetic `0x7d` transaction is protocol-generated. Users do not sign or submit it.

## Integration contract to publish

The public integration guide must document:

- transaction type: `0x7d`;
- the transaction is synthetic and always trails the user transactions;
- RPC serialization and required/omitted fields;
- `from`, `gas`, and `value` behavior;
- `input` contains the encoded `PostExecPayload`;
- affected user receipts expose `opGasRefund`;
- how to calculate pre-refund and post-refund gas usage;
- how explorers should display the transaction;
- how consumers should handle an unknown typed transaction without rejecting the containing block.

The schema is not considered frozen until the engineering-readiness checklist below is complete.

## Engineering readiness

- [ ] Publish and unhide the [SDM/`0x7d` RPC documentation](https://github.com/ethereum-optimism/optimism/blob/develop/docs/public-docs/chain-operators/guides/features/sequencer-defined-metering.mdx).
- [ ] Add an end-to-end test for `eth_getBlockByNumber(..., true)` returning the documented `0x7d` shape.
- [ ] Add an end-to-end test for `eth_getTransactionByHash`.
- [ ] Verify transaction and receipt cross-linking by block index.
- [ ] Add a backward-compatibility test using a stock `alloy_rpc_types_eth` decoder.
- [ ] Check representative viem/ethers decoders for unknown-transaction tolerance.
- [ ] Decide whether `l1Fee` and `l1GasUsed` on the `0x7d` receipt should be zero when `blobGasUsed` is zero, or document the current behavior.
- [ ] If standard decoders require signature fields, decide whether to flatten a zero signature similarly to deposit transactions.
- [ ] Produce example JSON responses and a real sample SDM block that can be shared with integration partners.

## Launch-critical outreach

### ✅ : Blockscout

- [x] Finalize the Blockscout backend changes. Starting point: https://github.com/nonsense/blockscout/pull/1
- [x] Open the corresponding Blockscout frontend change.
- [x] Share the integration contract and sample block with Blockscout.
- [x] Track review and merge status.
- [x] Verify an SDM block in a Blockscout deployment.

### Hosted explorers and developer tooling

- [ ] Contact Etherscan.
- [ ] Contact Tenderly.
- [ ] Contact OKLink.
- [ ] Contact Routescan/Superscan.
- [ ] Confirm whether each system:
- accepts blocks containing `0x7d`;
- indexes and displays the synthetic transaction;
- exposes `opGasRefund`;
- handles the adjusted `gasUsed` semantics;
- supports block and transaction tracing.

Internal communication tracker: https://github.com/ethereum-optimism/solutions/issues/769

## Secondary outreach

These are useful follow-ups but should not block closing the launch-critical portion of this issue:

- [ ] Phalcon Explorer / BlockSec
- [ ] Arkham
- [ ] Ethernal
- [ ] Otterscan, if support is still relevant

## Acceptance criteria

- [ ] A stable public `0x7d` integration guide is published.
- [ ] RPC and backward-compatibility tests enforce the documented schema.
- [ ] Example RPC responses and a sample SDM block are available.
- [ ] Blockscout backend and frontend work has been handed off upstream.
- [ ] Launch-critical hosted providers have been contacted.
- [ ] Responses, blockers, owners, and expected support dates are tracked.
- [ ] Any provider that will not support SDM before activation has a documented user impact and communication plan.
- [ ] Completion does not require every third party to merge support; it requires the contract, handoff, tracking, and explicit disposition.

## Related

- [SDM FMA](https://gist.github.com/nonsense/29013a36fbfd8b12ecd14276961402c8)
- [SDM external audit scope](https://gist.github.com/nonsense/10177901f413dd92aa71f271dc27dc58)
- [Ecosystem communication tracker](https://github.com/ethereum-optimism/solutions/issues/769)

Start with docs/public-docs/chain-operators/guides/features/sequencer-defined-metering.mdx and the SDM RPC implementation and integration-test entry points. Review the documented 0x7d shape, receipt fields, and decoder behavior before adding the listed end-to-end and compatibility coverage. Done means the guide, example responses, sample block, tests, and provider dispositions satisfy the acceptance criteria.

Written by the indexing model from the issue text.