The sampling profiler counted every frame occurrence in a stack for
cumulative statistics. For recursive functions appearing N times in a stack,
this meant counting N instead of 1, causing cumul% to exceed 100%. A function
recursing 500 deep in every sample would show 50000% cumulative presence.
The fix tracks seen locations per sample using a reused set, ensuring each
unique (filename, lineno, funcname) is counted once per sample. This matches
the expected semantics: cumul% represents the percentage of samples where a
function appeared on the stack, not the sum of all frame occurrences.
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