Target new P4D release in SDP regression test suite (release process Step 3, major releases only).
test_Upgrade.py's --to-version default: 25.2 -> 26.1, matching the P4D
version this SDP release ships against. --from-version stays at 25.1,
matching precedent (the doc's own worked example only ever moves
--to-version forward).
Agent: Claude Code, Model: Claude Sonnet 5 (claude-sonnet-5), operating as bot_Claude_Anthropic.