Window/day-based tape layout for continuous recording

Recording currently ships the session-scoped tape layout: one recording id per record/replay cycle. This is the right shape for per-run validation, but not for continuous recording of live traffic.

The day-windowed object-store layout (day-based windows, compaction, retention) is already designed but unbuilt. Build it when continuous recording — as opposed to per-run sessions — becomes the operating mode, and unify the session and window paths at that point (the same way graph mode is unified across paths today).

Tracked as a forward item; not blocking the current per-run validation flow.

Start by reading the existing session-scoped tape layout and the graph-mode path unification referenced in the issue. Then define the day-windowed object-store behavior for continuous recording, including compaction and retention. Done means continuous recording supports the window path and session and window handling are unified without blocking per-run validation.

Written by the indexing model from the issue text.