fix(sequencer): set own proposed checkpoint locally instead of via p2p loopback

[spalladino]

Motivation

The proposer relied on looping its own checkpoint proposal back through the p2p receive path to advance its local proposed-checkpoint tip before propagating. Under broadcastInvalidBlockProposal the broadcast checkpoint archive is deliberately corrupted, so the loopback handler's archive-based block lookup (getBlockData({ archive })) found nothing and retried until the next slot. By the time the proposer returned from broadcast, propagation had slipped past the p2p validator's stale window — producing intermittent failures (e.g. peers rejecting a late slot proposal).

Approach

The proposer's optimistic proposed-checkpoint tip is the proposer's own local state, so it is now set directly in the sequencer's checkpoint proposal job rather than via a p2p loopback. The job adds the proposed checkpoint to the archiver from local checkpoint data (block numbers and counts, never the possibly-corrupted broadcast archive) immediately before gossiping, failing closed if the local insert fails. Because every block is already added to the archiver's FIFO queue (and awaited) during block building, the checkpoint insert needs no retry. The notifyOwnCheckpointProposal loopback is removed entirely, so the path is identical whether p2p is enabled or not.

Changes

• stdlib: New ProposedCheckpointSink interface alongside L2BlockSink.
• sequencer-client: CheckpointProposalJob now pushes the proposed checkpoint to the archiver from local data before broadcast, gated on proposer pipelining and skipped when block-push is disabled (skipPushProposedBlocksToArchiver, fisherman mode); widened the sequencer/client l2BlockSource types to ProposedCheckpointSink.
• p2p: Removed notifyOwnCheckpointProposal from the P2PService interface, the libp2p and dummy services, and the P2PClient.broadcastCheckpointProposal call site (own proposals are still stored in the attestation pool before propagation).
• validator-client: The all-nodes own-proposal branch now skips validation and returns; removed the now-dead setProposedCheckpointFromBlocks and narrowed the archiver Pick.
• tests: Added job tests (push-from-local-data and order-before-gossip, abort-on-push-failure, no-push-when-pipelining-disabled, fisherman) and a proposal_handler own-proposal test; removed the obsolete libp2p loopback test and the e2e slash-test stub; widened affected mock types.