[ty] Use constraint sets as pending state in `SpecializationBuilder` (#24540)

This PR implements the bulk of the logic needed for
https://github.com/astral-sh/ty/issues/2799, by building up a
`ConstraintSet` in `SpecializationBuilder`. This constraint set holds
the "pending state" of the specialization being inferred. It will
eventually _replace_ the `types` hash map, which stores a simple mapping
from typevars to inferred types, and which is always updated by unioning
together types when we find multiple solutions for a typevar.

For now, we are building up both pending state representations, and
solving from the constraint set where we can. There are two main cases
where we must fall back on the old solver:

- Any specialization that involves a `ParamSpec`
- If the constraint set returns `Unsatisfiable` or `Unconstrained`

Follow-on PRs will address these remaining cases that use the old
solver; once they are all migrated over to the constraint set solver, we
will remove the `types` field.

Closes astral-sh/ty#2572
Closes astral-sh/ty#2959
Closes astral-sh/ty#3037
Closes astral-sh/ty#3203
Closes astral-sh/ty#3228
Closes astral-sh/ty#3428
Closes astral-sh/ty#3483