Plan to add s2 storage to browser events

[archandatta]

Note

Low Risk
Low risk because this PR only adds a planning/documentation markdown file and does not change any runtime code or behavior.

Overview
Adds a new design plan in plans/s2-storage.md describing a proposed durable browser events storage sink (S2-backed) alongside existing file and SSE outputs, including component responsibilities, batching/record-size constraints, shutdown sequencing, and testing approach.

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