[ty] Restore builder-local constraint subtype cache (#25844)
## Summary
PR #25786 originally cached repeated constraint-set subtype checks on
`ConstraintSetBuilder`. [During
review](https://github.com/astral-sh/ruff/pull/25786#discussion_r3383378278),
that cache was replaced with a Salsa-tracked query to allow reuse across
builders. On pytest-robotframework, the tracked query does not recover
the same performance: current main remains roughly 25% slower than the
commit immediately before #25778.
This restores the builder-local `FxHashMap` cache and routes the two
transitive-pivot subtype checks through it. The separate
type-variable-bound precheck from #25786 remains unchanged.
In 80 balanced, interleaved runs of pytest-robotframework using clean
profiling builds, the results were:
| Revision | Median | Paired change vs. pre-#25778 |
| --- | ---: | ---: |
| Pre-#25778 (`1676ec8013`) | 103.2 ms | — |
| Current main (`8f2caa6a04`) | 128.3 ms | +24.9% |
| This PR | 106.9 ms | +3.7% |
This PR is 16.9% faster than current main. All three revisions produced
byte-for-byte identical diagnostics. The full `ty_python_semantic` suite
(558 tests), focused Clippy, and repository hooks pass.
