Harden stack landing and product selector fallbacks

[cbusillo]

Summary

• drop descriptor/default fallback product choices when a DB-backed product with the same display label exists
• preserve product-backed choices by identity so real products with colliding labels are not hidden
• harden stack landing by persisting the root landing record before child disposition follow-up
• move the stack child disposition label into DB-backed merge-train policy instead of hardcoded service behavior
• preserve redirect-following deploy health probes

Verification

• pnpm --dir frontend validate
• deployed commit cae44a0 via Deploy Launchplane
• post-deploy /v1/health ok and /v1/products display-name counts are one per product
• uv run python -m unittest tests.test_merge_train_policy tests.test_merge_train_stack_collapse tests.test_service.LaunchplaneServiceTests
• uv run --extra dev ruff check control_plane/contracts/merge_train_policy.py control_plane/service.py tests/merge_train_policy_fixtures.py tests/test_merge_train_policy.py tests/test_service.py
• uv run --extra dev ruff format --check control_plane/contracts/merge_train_policy.py control_plane/service.py tests/merge_train_policy_fixtures.py tests/test_merge_train_policy.py tests/test_service.py
• uv run --extra dev mypy control_plane/contracts/merge_train_policy.py control_plane/service.py tests/merge_train_policy_fixtures.py tests/test_merge_train_policy.py tests/test_service.py
• actionlint .github/workflows/deploy-launchplane.yml .github/workflows/merge-train-runner.yml
• uv run python -m unittest

Notes

• yamllint still reports the repo's existing workflow line-length/truthy baseline; actionlint passes on the touched workflows.