[ty] Support `TypeVarTuple` and `Unpack` (#25240)
## Summary
closes: astral-sh/ty#156
closes: astral-sh/ty#1746
This PR adds initial support for `TypeVarTuple` and `Unpack` in ty.
### Supported
- Legacy `TypeVarTuple(...)` definitions from `typing` and
`typing_extensions`, including defaults, variance, and version
validation.
- PEP 695 `[*Ts]`, `Unpack[...]`, and starred unpack forms, with
diagnostics for invalid uses.
- Explicit specialization and substitution for generic classes and
aliases.
- Fixed and variable-length tuple construction, relations, display,
hover, and goto-type-definition.
- Legacy-solver inference from tuple arguments and returns,
constructors, and positional callable signatures.
- A `ParamSpec` following an unpacked variadic prefix.
### Representation and inference
A `TypeVarTuple` occupies one generic specialization slot and is
represented by a `TupleType`. Tuple specifications preserve a fixed
prefix and suffix around a variable middle and an unresolved / invalid
`TypeVarTuple` resolves as `tuple[Unknown, ...]`.
Callable inference constructs `ConstraintSet`s while matching positional
parameters in the signature relation. This keeps callable compatibility
and inference in the same loop instead of adding a separate
TypeVarTuple-specific path to `SpecializationBuilder`.
The work was briefly split into a [type-form
foundation](https://github.com/astral-sh/ruff/pull/26126) and generic
logic PR. I recombined them because the foundation alone caused
conformance and ecosystem regressions and did not represent complete,
independently reviewable behavior.
### Deliberate follow-ups
Call binding does not yet aggregate arguments matched to `*args` into a
`tuple`, or validate calls against concrete unpacked tuple annotations.
Per-argument inference is skipped because it would create incorrect
scalar mappings.
Nested tuple unpacking in callable parameter lists and some correlations
involving generic, overloaded, union, and inherited callables also
remain follow-ups. The ecosystem results show these callable cases
account for the main remaining false positives.
## Test Plan
- Added shared semantic coverage to the PEP 695 tests.
- Added legacy definition and specialization coverage.
- Kept the standalone `Unpack` coverage focused on its distinct syntax
paths.
- Reviewed the semantic suite, conformance results, and ecosystem
changes.
