Refactor continuity layer into orientation-cache modules

[Hmbown]

Scope

Factor session memory, command receipts, subagent/RLM handoff, compaction, and replay into explicit orientation-cache modules.

Acceptance

• Define module boundaries for Distiller, Cartographer, Evictor, storage, receipt ingestion, and handoff export/import.
• Command/tool/subagent receipts enter continuity through one ingestion path with source links and detail handles.
• Compaction and resume consume the same orientation entries instead of bespoke summaries.
• Tests cover preserving a source-linked receipt through compaction/resume and evicting stale low-value entries first.