[ty] Support forward references in simple stub assignments

[carljm]

Summary

• Resolve forward references in directly evaluated simple stub assignments using the existing deferred-expression semantics, covering class aliases, union aliases, and ordinary values.
• Preserve at-use resolution for already-bound names so aliases of conditionally overloaded functions retain their complete overload sets; keep runtime-module assignments and specialized call handling unchanged.
• Correct NumPy 2.5.1's _dtype = dtype forward alias so ndarray.view(np.uint32) selects the dtype overload and infers the proper ndarray dtype.

Fixes astral-sh/ty#3967.
Also fixes astral-sh/ty#4208.

Test plan

• Added mdtests for forward and chained class aliases, union aliases, value aliases, builtins-stub forward references, generic-bound overload selection, existing bindings, self-reassignment, eager .py diagnostics, and aliases of conditionally overloaded methods.
• All 494 semantic mdtests and parser, linter, typeshed, and semantic panic-corpus tests pass.
• Replayed the original NumPy reproduction and affected Porcupine and Bokeh projects: overloaded method calls remain valid while the intended NumPy-related diagnostic improvements are preserved.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 96.96%. The percentage of expected errors that received a diagnostic held steady at 92.96%. The number of fully passing files held steady at 106/133.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
invalid-argument-type	0	2	0
unsound-return-statement	0	1	1
Total	0	3	1

Raw diff:

bokeh (https://github.com/bokeh/bokeh)
- src/bokeh/util/hex.py:235:27 error[invalid-argument-type] Method `__getitem__` of type `Overload[(key: _ArrayInt_co | tuple[_ArrayInt_co, ...], /) -> ndarray[_AnyShape, list[tuple[str, Any]]], (key: SupportsIndex | tuple[SupportsIndex, ...], /) -> Any, (key: _ToIndices, /) -> ndarray[_AnyShape, list[tuple[str, Any]]]]` cannot be called with key of type `Literal["q"]` on object of type `ndarray[_AnyShape, list[tuple[str, Any]]]`
- src/bokeh/util/hex.py:235:45 error[invalid-argument-type] Method `__getitem__` of type `Overload[(key: _ArrayInt_co | tuple[_ArrayInt_co, ...], /) -> ndarray[_AnyShape, list[tuple[str, Any]]], (key: SupportsIndex | tuple[SupportsIndex, ...], /) -> Any, (key: _ToIndices, /) -> ndarray[_AnyShape, list[tuple[str, Any]]]]` cannot be called with key of type `Literal["r"]` on object of type `ndarray[_AnyShape, list[tuple[str, Any]]]`
- src/bokeh/palettes.py:1529:12 warning[unsound-return-statement] Unsound return statement: `tuple[Unknown, ...]` is not a subtype of `Palette`

meson (https://github.com/mesonbuild/meson)
- mesonbuild/compilers/mixins/visualstudio.py:392:16 warning[unsound-return-statement] Unsound return statement: `list[str] | list[str | Divergent]` is not a subtype of `list[str]`
+ mesonbuild/compilers/mixins/visualstudio.py:392:16 warning[unsound-return-statement] Unsound return statement: `list[str] | list[Divergent]` is not a subtype of `list[str]`

Full report with detailed diff (timing results)