docs: stateless chat client live KV warm recipe

[paride-cmd]

Summary

• Adds a README subsection under Disk KV Cache for keeping the live session warm from a stateless chat/completions client (exact assistant replay, `max_tokens: 0` prewarm, `cached_tokens` witness).
• Notes that cold disk checkpoints trim to `kv_cache_chat_anchor_pos`, which is the usual disk hit when a client cannot replay.

Fixes #816

Test plan

• Read the new subsection against the measurements and playbook in Stateless chat clients never extend the live KV session on Flash/Metal: measurements, the anchor that saves them, and the replay recipe that fixes it #816
• Confirm cold-checkpoint bullet mentions the chat anchor fallback

AI usage disclosure: YES — assisted by Cursor agent; change is docs-only.

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