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8727a7b1 - Fix stack overflow with recursive generic protocols (depth limit) (#21858)

Fix stack overflow with recursive generic protocols (depth limit) (#21858) ## Summary This fixes https://github.com/astral-sh/ty/issues/1736 where recursive generic protocols with growing specializations caused a stack overflow. The issue occurred with protocols like: ```python class C[T](Protocol): a: 'C[set[T]]' ``` When checking `C[set[int]]` against e.g. `C[Unknown]`, member `a` requires checking `C[set[set[int]]]`, which requires `C[set[set[set[int]]]]`, etc. Each level has different type specializations, so the existing cycle detection (using full types as cache keys) didn't catch the infinite recursion. This fix adds a simple recursion depth limit (64) to the CycleDetector. When the depth exceeds the limit, we return the fallback value (assume compatible) to safely terminate the recursion. This is a bit of a blunt hammer, but it should be broadly effective to prevent stack overflow in any nested-relation case, and it's hard to imagine that non-recursive nested relation comparisons of depth > 64 exist much in the wild. ## Test Plan Added mdtest.