refactor(stdlib): carry the proposed tip header on chain-proposed events

[nchamo]

Why we are doing this

Follow-up to #25089 in the PXE↔node RPC-reduction line. When PXE tracks the proposed chain tip it anchors on the tip's header, but the block stream's chain-proposed event only carried the block id, so the PXE handler had to fetch the header back from the node on every tip movement.

Our fix

chain-proposed now carries the tip's BlockHeader as a required payload, so consumers anchor on the event without a fetch of their own. The stream sources the header from whatever the pass already has: the delivered tip block in block mode, a prefetch that runs in parallel with the reorg walk-back in tips-only mode, or an on-demand by-hash read as a fallback. If the header cannot be obtained the pass aborts and retries on the next poll, so tier events never get ahead of a proposed tip the consumer never received.

Metrics

Measured on the key_flows transfers benchmark with a counting wrapper between the benchmarking wallet and the in-process node, applied to both base and this branch. Per-flow calls are unchanged; the saving is one serial fetch (round trip) per tip movement.

Flow	RPC calls	Round trips
ecdsar1+transfer_0_recursions+sponsored_fpc	35 → 35	26 → 25 (−3.8%)
ecdsar1+transfer_1_recursions+sponsored_fpc	37 → 37	23 → 21 (−8.7%)
ecdsar1+transfer_0_recursions+private_fpc	56 → 56	34 → 33 (−2.9%)
ecdsar1+transfer_1_recursions+private_fpc	56 → 56	32 → 31 (−3.1%)
whole suite (both wallets, incl. setup)	460 → 459	333 → 320 (−3.9%)

[nventuro]

Are we ok with dropping this error scenario?

[nchamo]

I think so. We moved it into a warn in l2_block_stream.ts:

this.log.warn(`No header for the proposed tip; aborting this sync pass`, {
    blockNumber: sourceTips.proposed.number,
    blockHash: sourceTips.proposed.hash,
});

The idea is that we can retry again and maybe the node error was only temporary. We could potentially remember attempts and fail if we tried too many times, but it feels like an overkill to me

What do you think we should do?

[AztecBot]

✅ Successfully backported to backport-to-v5-next-staging #25135.