[epic] Reworking solver internals to avoid simple Var pinning #3123

Describe the Bug

This issue tracks a single unit of work - improving Pyrefly's constraint solver so that we can accumulate bounds on Vars and otherwise solve them in an intelligent way where eager pinning cannot - to serve as a parent for multiple issues mapping to specific behaviors and bug reports.

The overall context is that Pyrefly is evolving from a solver that simply pins Vars immediately on all comparison to one that can do proper bounds-based constraint solving, and we know of multiple usability issues that are actually all tied in with the same design problem:

• eager pinning of quantifieds in calls (Function invocation pins type of arguments too early #105 )
• problems with union contextual typing (Contextual typing works only with the first element in a union #793 )
• problems with AnyStr in calls (False positive bad-argument-type when calling a method with AnyStr #2644 )

Those issues track different end-user behavior, but this issue tracks the actual project to solve them.

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(Only applicable for extension issues) IDE Information

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