Preserve stack collapse record compatibility

[cbusillo]

Summary

• load previously stored stack-collapse plans that recorded mutations in root-to-leaf order
• rebuild missing child disposition guards from already executed mutation results instead of stale entry heads
• cover both compatibility paths with focused contract tests

Verification

• uv run python -m unittest tests.test_merge_train_stack_collapse
• uv run --extra dev ruff check control_plane/contracts/merge_train_stack_collapse.py tests/test_merge_train_stack_collapse.py
• uv run --extra dev ruff format --check control_plane/contracts/merge_train_stack_collapse.py tests/test_merge_train_stack_collapse.py
• uv run --extra dev mypy control_plane/contracts/merge_train_stack_collapse.py tests/test_merge_train_stack_collapse.py
• uv run python -m unittest tests.test_service.LaunchplaneServiceTests tests.test_filesystem_store tests.test_postgres_store