fix(pageserver): prevent stack overflow in count_deltas

[AesaKamar]

Summary

This PR fixes a stack overflow in the pageserver's LayerMap::count_deltas function (resolving #3711).

1. Iterative Stack Machine: Converted the recursive DFS traversal of the historical layer map into an iterative, stack-based algorithm utilizing a heap-allocated task stack. This prevents call stack exhaustion when evaluating very deep delta layer maps (such as during active GC and compaction).
2. Regression Test: Added test_count_deltas_stack_overflow in layer_map.rs which verifies that evaluating a stack of 10,000 delta layers completes successfully without crashing.

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P.S. This is another contribution following up on my first PR! The iterative version runs safely and extremely fast (the regression test passes in <0.3s). I would love any feedback on the implementation!