feat(sequencer): build optimistically across pruning epoch boundary

[spalladino]

Motivation

At a pruning epoch boundary, today's canProposeAtTime simulation pre-emptively reverts when an unproven epoch's deadline is about to expire — even if the proof lands seconds later. The slot is silently skipped. This loses a checkpoint window for no good reason: the publisher's preCheck right before L1 submission is the authoritative gate.

Similarly, the simulation overrides applied to the preCheck flight due to pipelining (as in overriding the pending chain with the last mined slot) meant that we were silently missing the case where the epoch prune did trigger, so we were sending the tx and reverting. This is fixed by having different plans for the first simulation and the right-before-submission simulation. That said, sequencer publisher checks are a bit convoluted now, so I'm making a pass at them to try and simplify in a later PR.

Approach

Apply a proven-override at the three pre-submission simulation sites (canProposeAt, the globals builder, and enqueue-time validateCheckpointForSubmission) that forces pending == proven so STFLib.canPruneAtTime short-circuits to false. Submission's preCheck runs without the override against real L1 state and decides whether to actually send. A new structured preparing-checkpoint sequencer event surfaces the override/parent state for tests. Tip storage now goes through a single makeChainTipsOverride to avoid same-slot state-diff clobbering.

Changes

• archiver: isPruneDueAtSlot(slot) on L2BlockSource replicates STFLib.canPruneAtTime locally (no L1 RPC).
• ethereum: RollupContract.makeChainTipsOverride({pending?, proven?}) writes a single combined state-diff and guards proven > pending. forPendingCheckpoint(n) → withChainTips({pending?, proven?}) on the simulation overrides builder.
• sequencer-client (publisher): enqueueProposeCheckpoint accepts preCheckSimulationOverridesPlan separately from simulationOverridesPlan; the preCheck closure uses it (no fallback) so the parent / proven overrides never reach pre-send validation.
• sequencer-client (sequencer): applies the proven override at the canProposeAt site, plumbs it through prunePending to CheckpointProposalJob so the globals builder and enqueue-time validation see it. New pauseProposingForSlots test-only config.
• sequencer-client (events): new preparing-checkpoint event with targetSlot, checkpointNumber, hadProposedParent, provenOverride.
• ethereum (test infra): Delayer.pauseNextTxUntil* accept a per-call timeout to support boundary tests that need to wait > 180s.
• end-to-end (new tests): epochs_proof_at_boundary.parallel.test.ts covers smoke + four boundary scenarios — proof lands during pipeline sleep; proof lands well before deadline; proof never lands (with parent); proof lands / never lands without proposed parent — using structured events and retryUntil rather than log greps.
• stdlib + interfaces: schemas and configs updated for the new RPC method and the new sequencer config knob.