refactor: use add and drop constraint actions

[Tomoscorbin]

Summary

• rename primary- and foreign-key Set* actions and phases to Add*
• carry desired constraints for additions and observed catalog occurrences for drops
• give constraint actions operational identity through desired/catalog names, including the primary-key name rendered by CreateTable
• require compiled plans to retain the exact source action objects

Why

Constraint values intentionally compare by relational definition, independent of lifecycle names. Executable actions still need to compare differently when they would render different SQL. Keeping that distinction in the action layer preserves structural constraint equality without losing operational identity.

Stack

• Base: refactor: model desired and observed constraints #355 (feature/model-constraint-lifecycle)
• This PR: PR 2 of the constraint lifecycle redesign
• Review and merge refactor: model desired and observed constraints #355 first. Then rebase this branch onto, and retarget this PR to, feature/constraint-lifecycle-redesign-plan.

Validation

• uv run pytest -q — 1,268 passed, 78 deselected; 97.16% coverage
• uv run mypy src
• uv run ruff check .
• uv run ruff format --check .
• uv run lint-imports
• uv run --group docs sphinx-build -W -b html docs /tmp/delta-engine-constraint-action-docs
• git diff --check