[ty] Cache protocol receiver binding (#27301)
## Summary
Protocol-member compatibility repeatedly specializes the same callable
for the same runtime receiver and typing `Self`, rebuilding signatures
and constraint sets on each structural comparison.
- Add a private Salsa-tracked protocol receiver-binding cache keyed by
the callable, receiver type, and `Self` type.
- Route all four existing protocol compatibility binding sites through
the cache, including protocol-to-protocol member comparisons.
- Preserve distinct receiver and `Self` bindings and reuse `apply_self`
when they are identical.
This is independently useful on `main` and becomes even more relevant
with #27267.
## Performance
DateType on clean `main` and this PR, with 40 samples per version:
| Version | Estimate | 95% confidence interval |
| --- | ---: | ---: |
| Baseline | 38.701 ms | 38.336–39.125 ms |
| Protocol receiver cache with fixed-point recovery | 33.887 ms |
33.742–34.033 ms |
The change is **12.4% faster**, with non-overlapping confidence
intervals.
## Test plan
- Add a minimized mdtest for a generic protocol whose receiver-specific
method and classmethod produce a recursive receiver-binding query;
verify that the expected return-type diagnostic is reported instead of a
Salsa panic.
- Reproduce the exact ecosystem-analyzer configuration for DateType,
scipy-stubs, and trio, and verify that all three projects produce
byte-for-byte identical diagnostics to `main`.
