Caplin: persist Gloas envelope metadata for historical serving

## Context

In Gloas (EIP-7732), the execution payload is delivered in a separate `SignedExecutionPayloadEnvelope` rather than embedded in the beacon block. Caplin currently stores these envelopes only in the fork graph's temporary disk cache, which is pruned ~3 epochs after finalization (`fork_graph_disk.go:Prune`).

## Gap

The P2P handlers (`ExecutionPayloadEnvelopesByRoot`, `ExecutionPayloadEnvelopesByRange`) and the Beacon API (`GET /eth/v1/beacon/execution_payload_envelope/{block_id}`) read envelopes via `forkChoiceReader.ReadEnvelopeFromDisk`. After pruning, these return nothing — even though the protocol serve range (`MIN_EPOCHS_FOR_BLOCK_REQUESTS`, ~33k epochs on mainnet) is far larger than the retention window.

The EL side has `ExecModule.GetPayloadBodiesByHash/Range` (wired into `ChainReaderWriterEth1`) which returns transactions, withdrawals, and BAL bytes — but not the full envelope. The missing fields are `ExecutionRequests`, `BuilderIndex`, `BeaconBlockRoot`, `ParentBeaconBlockRoot`, and the builder `Signature`.

## Affected consumers

- **P2P**: `executionPayloadEnvelopesByRootHandler` / `executionPayloadEnvelopesByRangeHandler` — cannot serve envelopes outside the fork graph retention window
- **Beacon API**: `GET /eth/v1/beacon/execution_payload_envelope/{block_id}` — returns 404 for pruned blocks (relevant for archive nodes)

All other envelope consumers (chain tip sync, forward sync, block production, payload attestation) only need recent data and are unaffected.

## Possible direction

Persist only the envelope metadata (~200 bytes–1 KB per block: `ExecutionRequests`, `BuilderIndex`, `BeaconBlockRoot`, `ParentBeaconBlockRoot`, `Signature`) in the beacon DB. At serve time, reconstruct the full `SignedExecutionPayloadEnvelope` by combining persisted metadata with payload body from `ExecModule.GetPayloadBodiesByHash`. This avoids duplicating the bulk of the data (transactions, withdrawals, BAL) that the EL already stores.

## Priority

**Not urgent.** Gloas is in devnet — node count is small, sync distances are short, and the fork graph cache is sufficient for now. This issue tracks the gap for when testnet/mainnet timelines get closer.