[SR] Buffer mode improvements

[romtsn]

📜 Description

Resolved some TODOs:

• Align next segment timestamp with the end of the buffered segment, when converting from buffer mode to session mode
• Persist buffer replay type for the entire replay when converting from buffer mode to session mode
• Rework how screenAtStart is stored - now we retrieve current screen name for each frame and store it in the replay cache, and then when we have to rotate the cached frames, we always get the correct screen at start of the buffered replay

💡 Motivation and Context

part of getsentry/sentry#74441

💚 How did you test it?

manually + automated

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	351.63 ms	400.39 ms	48.76 ms
Size	1.70 MiB	2.34 MiB	659.61 KiB

[stefanosiano]

lgtm!