Fix memory leak in overload resolution

[samschlegel]

Summary

Replace fresh partial variable allocation with snapshot/restore during overload evaluation.

The freshen_partial_vars approach created new solver variables for every overload candidate. I think these accumulated in the solver's variable map and were never freed, causing unbounded memory growth in certain codebases (e.g. 90MB -> 49GB on a play_scraper repro.) This isn't reproducible on numpy or other large projects, so the specific trigger is unclear.

Instead, snapshot partial var state before evaluating overloads and restore after each attempt. This prevents losing overloads from pinning partial vars without allocating new variables.

Fixes #3088

Test Plan [WIP]

• Run cargo tests
• Run test.py
• Understand underlying cause
• Minimize repro example
• Get a failing test working

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