generics: specify TypeVar-default inference when parameter default matches

[ashishpatel26]

Summary

Fixes #2213.

The spec's "Function Defaults" section previously left all generic-function TypeVar default semantics unspecified. However, there is a concrete, well-defined case that can and should be specified:

When a TypeVar S with default D is used as the type of exactly one parameter p, and p's default argument value is assignable to D, calling the function without p must infer S = D.

Additionally, the function definition is valid in this case: the default argument is checked against D (the TypeVar's default), not against the free TypeVar S itself.

This is already the behavior of pyright (and pyrefly, pycroscope) — the spec is simply catching up.

Changes

• docs/spec/generics.rst — "Function Defaults" section: keep the general caveat but carve out and specify this concrete case with an example
• conformance/tests/generics_defaults.py — new test section with a Getter[T] class whose .get() has S=None default
• Checker TOMLs updated:
  • pyright, pyrefly, pycroscope — pass (no change to Pass status)
  • mypy, zuban — incorrectly reject the function definition → Partial
  • ty — incorrectly rejects the function definition, but correctly infers return types → Partial

Checker behaviour at a glance

Checker	def get(..., default: S = None)	getter.get() → str | None
pyright	✅ valid	✅
pyrefly	✅ valid	✅
pycroscope	✅ valid	✅
mypy	❌ rejects (false positive)	✅
zuban	❌ rejects (false positive)	✅
ty	❌ rejects (false positive)	✅

Test plan

• conformance/src/main.py --report-only produces a clean results.html
• validate_results.py passes (Fail results have conformant field)