Guard stack-collapse landing evidence

[cbusillo]

Summary

• validate stack-collapse evidence before landing the root PR
• execute stack collapse leaf-to-root so the root receives every child change before admission
• re-read child PR heads after close so close races are recorded as stale evidence

Verification

• uv run python -m unittest tests.test_merge_train_stack_collapse tests.test_merge_train_github tests.test_service.LaunchplaneServiceTests.test_merge_train_stack_collapse_service_admits_executed_root_only tests.test_service.LaunchplaneServiceTests.test_merge_train_batch_landing_service_closes_stack_children_after_root_lands tests.test_service.LaunchplaneServiceTests.test_merge_train_batch_landing_service_validates_stack_before_root_merge tests.test_service.LaunchplaneServiceTests.test_merge_train_stack_collapse_service_rejects_admit_when_root_head_moves tests.test_service.LaunchplaneServiceTests.test_merge_train_stack_collapse_service_rejects_admit_when_policy_digest_changes
• uv run --extra dev ruff check --diff control_plane/contracts/merge_train_stack_collapse.py control_plane/merge_train_github.py control_plane/service.py tests/test_merge_train_stack_collapse.py tests/test_merge_train_github.py tests/test_service.py
• uv run --extra dev ruff check control_plane/contracts/merge_train_stack_collapse.py control_plane/merge_train_github.py control_plane/service.py tests/test_merge_train_stack_collapse.py tests/test_merge_train_github.py tests/test_service.py
• uv run python -m unittest
• targeted PyCharm inspection on changed files; existing project warnings remain unrelated