Conversation memory with demotion, compaction, and active window management.

This crate provides:

• [ConversationMemory]: trait for loading/appending/clearing conversation messages
• [DemotionHook]: trait for demoting old messages to long-term storage
• [Compactor]: trait for summarizing old messages into a compact form
• [ActiveWindow]: manages the short-term context window with policy-based trimming
• [MemoryPolicy]: configures window size, token budget, and retention rules

Lifecycle

New message → ActiveWindow::push()
  → if over token limit → trim()
    → DemotionHook::demote()  (save to long-term storage)
    → Compactor::compact()    (summarize and inject back)