solana-indexer: advance the finalized watermark from slot statuses

[squadgazzz]

Description

The yellowstone subscription only delivered confirmed slot statuses, so the indexer never saw finality: solana.indexer_state.finalized_slot and solana.chain_tip had no writer, and the last-indexed watermark only advanced when a settlement flushed. On an idle chain that stalls the resume point until it falls out of the provider's replay window (observed 825 slots behind on devnet), turning every restart into a live-tip resubscribe with a gap.

This is the first half of BE-203. The rollback cascade for rows above the finalized watermark follows in its own PR.

Changes

• The slot filter subscribes to every status transition. The ingester forwards confirmed statuses (flush signal and tip counter) and finalized ones (watermark), and drops the rest: a status ahead of the transaction stream's commitment must not flush a buffer whose transactions are still in flight.
• finalized_slot advances from finalized statuses, monotone and update-only.
• The last-indexed watermark advances on quiet slots up to the flush hold-back, so the resume point tracks the stream instead of the last settlement.
• solana.chain_tip is dropped (V2 migration). The quiet-slot advancement makes indexer_state.slot move every confirmed slot, so the tip row would trail it by the 2-slot hold-back and add no signal: freshness checks read the watermark.

How to test

New unit tests and ignored postgres tests, including the pipeline test driving finalized and quiet-slot statuses end to end.

Related issues

BE-203

[linear-code]

BE-203