[MetaROR] Ch1 Background — author revisions

Author revisions for **Chapter 1 (Background)** from the MetaROR reviews (R1 Hannah Fraser, R2 Vittorio Iacovella, R3 Wolf Vanpaemel) and internal notes.

- [x] Improve the replication-rate discussion — currently rests on a single, narrow (education) paper. Broaden the evidence base and caveat the estimate. (Lukas) (in #42)

**Related existing issues** (whoever takes this owns these too):
- #12 Incorporate new findings about replication success
- #7 Revise introductory chapters based on Breznau et al.'s Teaching Companion

**Handled in other issues:**
- Figure 1.1 vertical dashed line ~2010 → *Automatable tasks*
- Merge Chapter 1 into Chapter 2 → *Structural reorganisation & scope*

Start by reviewing the related work in #42, #12, and #7, since the listed replication-rate revision is already noted as handled in #42 and the remaining scope is not specified here. Confirm which Chapter 1 changes remain, then ensure the agreed author revisions are incorporated and reviewed.

Written by the indexing model from the issue text.