Differential flamegraphs are incorrectly scaled across sampling rates

Differential flamegraphs compare raw sample counts after scaling the baseline by sampled wall time. This makes identical profiles look very different when the sampling rates differ.

For example, using a 1 kHz baseline and profiling the same workload at 100 Hz reports around -90% for unchanged leaf functions. Using the same rate for both profiles reports approximately 0%.

We should normalize the sample counts consistently and use the sampling interval only when converting samples to time for display.

Linked PRs

• gh-154082

先使用 1 kHz 和 100 Hz 的 profile 重現 issue 中描述的 differential flamegraph 比較，然後追蹤 sample 計數在哪裡進行縮放，以及 sampling interval 在哪裡將 sample 轉換為顯示時間。當不同 sampling rate 下的相同 profile 比較結果約為 0%，且顯示時間轉換仍然正確時，即可視為完成。