docs(plan): add PR split strategy for safe work preservation

Phase 4 documents how to split PR #859 into:
- Base PR: infrastructure + minimal proof-of-work (1 RST, 1 MyST, 1 notebook)
- Stacked PR: rich graphviz examples across all docs

Includes safe workflow with backup tag before trimming.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lmeyerov