refactor: model desired and observed constraints

[Tomoscorbin]

Summary

• split key constraints into Desired* declarations and Observed* catalog occurrences
• use lifecycle-specific desired_name and catalog_name fields
• define primary- and foreign-key equality and hashing by relational definition across lifecycle variants
• share normalization and identity through private functions instead of additional constraint-shaped base classes
• make desired and observed tables reject constraint values from the wrong lifecycle
• make the information-schema reader the sole production constructor of observed constraints
• reconcile primary keys with direct equality and foreign keys with a deterministic set-based one-to-one match

Why

Constraint names have different meanings before and after creation. A desired name is a creation preference; an observed name is a physical catalog handle. Treating either as relational identity creates false drift and replacement work.

This PR makes desired_key == observed_key express the reconciliation question directly. The four lifecycle values are the complete constraint vocabulary; private functions hide their shared mechanics without introducing another constraint-like entity.

Behavior and compatibility

This intentionally changes name-only drift: a matching catalog definition is adopted regardless of its catalog name. Structural differences still produce drop-and-set actions.

Public declarations still generate the existing default desired names, and newly created constraint SQL remains named exactly as before. Optional names, Databricks-assigned naming, and the Set* to Add* action redesign remain in later PRs.

This is PR 1 of the constraint-lifecycle redesign and is stacked on feature/constraint-lifecycle-redesign-plan.

Validation

• uv run pytest -q — 1,258 passed, 78 deselected; 97.14% 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-pr355-docs
• git diff --check