refactor(node/sync): tidy backfill, require store_view, fix range watermark (leanEthereum#1109)

Clarity, modern-Python, and one liveness fix in BackfillSync.

Documentation:
- Trim the banner-essay module and class docstrings and the verbose method
  Args/Returns blocks to the project rules, keeping the genuine rationale
  (depth cap bounds an attacker-claimed ancient parent; head-not-finalized
  gap floor; authoritative-only watermark) glued to its code.

Structure:
- Make store_view required and drop its dead None mode; no production caller
  passed None, and the degraded branch only served tests.
- Inline the two single-use fetch helpers into their batch loops, turning
  their per-batch return into continue so the loop still advances.
- Batch both fetch loops with itertools.batched instead of manual index and
  slot-offset arithmetic.

Watermark liveness fix:
- Advance the range watermark only on an authoritative answer (the peer
  claimed the range, or returned blocks). An empty reply from a fallback
  peer no longer strands a range that another peer can still serve.
- Make the watermark Slot | None so genesis slot 0 is fetchable; the old
  Slot(0) sentinel could not tell "covered" from "nothing covered".
- Cover the fix with a regression test; make the test peer authoritative.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tcoratger