server: fix pos_min gate + improve ring buffer log messages

- Fix inverted pos_min gate in create_checkpoint() that caused checkpoint
  search to skip on warm slots. The condition `it->pos_min < pos_min_thold`
  was backwards -- should be `>=` to find checkpoints that cover the LCP
  position. This was silently forcing full re-prefill on warm slots.

- Improve ring buffer log message readability in create_checkpoint() and
  deferred_create_final_checkpoint(). Updated 4 log messages to be more
  concise and follow the same format as other log messages:

  - "kv ring buffer, evicted oldest (size=X MiB)"
  - "kv ring buffer, pushed mid-prompt to slot N/M (pos_min=X size=Y MiB)"
  - "kv ring buffer, recycled slot N/M (dropped pos_min=X size=Y MiB)"
  - "kv ring buffer, recycled/pushed final checkpoint to slot N/M (pos_min=X size=Y MiB)"

  Removed redundant pos_max and n_tokens fields, shortened the message
  prefix from "[ring buffer] appended new slot at back (buffer=9/16)" style
  to "kv ring buffer, ..." style.