perf(replays): Optimize memory usage and deserialization performance

[cmanallen]

msgspec allows us to partially deserialize a JSON payload. By removing the requirement to deserialize full and incremental snapshots, we a significantly increase the performance of the deserialization and significantly reduce the memory consumption.

This is particularly important as our consumer suffers from memory usage issue and our single-threaded processing logic is a bottleneck to throughput improvements. Experiments demonstrated a 10x performance uplift and memory usage declined to, essentially, nil.

There is a downside. We can no longer read data from incremental snapshots or full snapshots. To do so would require allocating memory and defeat the purpose of this PR. Presently we don't do anything with those events except log some canvas events which, I think, are unimportant enough we can remove them.

Full plan: https://www.notion.so/sentry/Q4-25-Recording-Consumer-Improvements-28c8b10e4b5d803cb212e19aebefee81

Related: getsentry/pypi#1695

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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