Conversation cache layout, anchoring, TTL, and LTM pin invariants

Use the 1h conversation-cache TTL. The distilled prefix occupies transformed positions [0,1]; durable knowledge deltas stay near the tail, and orphan removal never touches the prefix. Meta passthrough remains uncached. Keep CACHE_ANCHOR_MIN_MESSAGES >= 2 * CACHE_ANCHOR_STEP: below that, the quantized anchor index remains zero and the intermediate anchor never fires. Persisted ltmCacheText must equal ltmPinText; the pin is baseline-only metadata and is never sent upstream. ltm.forSession() must not bust system[2] when the selected entries are unchanged, except for byte-identical re-anchoring. Independently rendering cache and pin text, moving deltas forward, or caching passthrough destabilizes bytes and can cause prefix drift or cache spirals.