Session 'context collapse' tracking replaced with 'memory mode' stamps

What

• A new memory-mode session-log entry type records when memory mode turns on or off, along with a reason (paused, session_shape, or server_off) and the account involved; these entries are appended to a per-session ledger and merged with an "accumulate" policy.
• The old marble-origami-commit/snapshot/reset tracking, and its contextCollapseCommits/contextCollapseSnapshot fields, have been removed entirely from session state, sync broadcasts, and serialization, replaced by these memory-mode entries and a new memoryModeStamps array threaded through session loading, resuming, and merging.
• Forking a conversation now carries these memoryModeStamps over into the new forked session's metadata.

Why

This replaces an older mechanism for tracking memory/context state with a clearer one that records why memory mode changed, and preserves that history when a session is forked.