test(e2e): anchor e2e_amm PXE to checkpointed tip under pipelining

[AztecBot]

Why

PR #23253 was dequeued (4th attempt) when merge-queue-heavy caught an e2e_amm.test.ts setup tx getting dropped by a pipelining-driven chain prune. CI log: baec5a7453c20089.

The wait-for-parent gate in CheckpointProposalJob.waitForValidParentCheckpointOnL1 (sequencer-client/src/sequencer/checkpoint_proposal_job.ts:398) should have blocked the discard, but it didn't — because a TestDateProvider time warp from AnvilTestWatcher.syncDateProviderToL1IfBehind landed between the two epochCache reads in Sequencer.work (sequencer.ts:217-218) and broke the pipelining invariant.

step	wall-clock	nowSeconds	result
1st getEpochAndSlotInNextL1Slot (slot)	≈14:34:32.385 (pre-warp)	1778942079	next L1 ts 1778942080 → slot 18
(warp at 14:34:32.390 sets offset 7611 → 7610)
2nd getTargetEpochAndSlotInNextL1Slot (targetSlot)	≈14:34:32.395 (post-warp)	1778942080	next L1 ts 1778942084 → slot 19 → +offset=1 → targetSlot 20

Logged confirmation (gap = 2 instead of 1):

14:34:32.612  Preparing checkpoint proposal 19 for target slot 20 during wall-clock slot 18
              {nowSeconds=1778942079, slot=18, targetSlot=20, …}

With slotNow = 18, the gate at checkpoint_proposal_job.ts:402 waits on waitForSyncedL2SlotNumber(slotNow). The archiver had already synced past slot 18 — the wait returns immediately, far too early to see parent ckpt 18 (which lands four seconds later at 14:34:36). The gate then sees checkpointedNumber=17, parentCheckpointNumber=18, declares the parent absent, and discards. Slot 20 expires uncheckpointed, archiver prunes blocks 19/20, the inflight setup tx anchored to block 19 dies with Block header not found.

Full timeline + log evidence: https://gist.github.com/AztecBot/4863d10084dd20587bffcc43fd61dfee

What

Scoped, test-only — per direction from Santiago. The previous "make checkpointed the global PXE default" approach is reverted; only e2e_amm is opted in:

-    } = await setup(4, { ...PIPELINING_SETUP_OPTS }));
+    } = await setup(4, { ...PIPELINING_SETUP_OPTS }, { syncChainTip: 'checkpointed' }));

The PXE option exists already (yarn-project/pxe/src/config/index.ts, added in 75df5b5d44). This is the same approach every other pipelining-aware test uses (e2e_p2p/*, e2e_epochs/*, e2e_slashing/attested_invalid_proposal). It anchors inflight txs to the L1-confirmed tip so prunes on the proposed tip can't invalidate them.

PIPELINING_SETUP_OPTS is left untouched — the pipelining migration of e2e_amm in #23275 stays.

Recommended follow-up (separate PR)

The real bug is the race in Sequencer.work. Worth fixing properly:

• Snapshot the time once. Add EpochCache.getCurrentAndTargetSlotInNextL1Slot() that returns {slot, targetSlot, epoch, targetEpoch, ts, nowSeconds} from a single dateProvider.nowInSeconds() read; replace the two-call site in Sequencer.work. Pipelining offset is a constant, so deriving targetSlot = slot + offset from the same snapshot is trivial.
• Defensive: wait on targetSlot - 1. waitForValidParentCheckpointOnL1 should key off the parent's expected build slot (targetSlot - 1) instead of slotNow, so the gate is robust even if the invariant is broken upstream.

These aren't in this PR because they touch sequencer production code and want their own review; the test-side workaround unblocks the merge-train without changing the global PXE default.

Test plan

The failure requires merge-queue-heavy's 10-grind L1 contention to surface reliably (single dev box can't reproduce). Change is a single-arg addition; TS-trivial.

Analysis: https://gist.github.com/AztecBot/4863d10084dd20587bffcc43fd61dfee

ClaudeBox log: https://claudebox.work/s/166e664eab264b04?run=3