FastStrata v0.6.0

A long conversation comes back after the model was stopped. The FastStrata Gateway stops the model when its app
closes, at Stop and when it swaps the model, and the engine kept its conversations in RAM only, so the next prompt of
a long conversation was read from its first token. Now the engine keeps them on disk too, and its next start takes them
up: on the 6 GB test laptop the next turn of a 70K conversation after a restart started after 7.2 s where 83.0, of a
40K one after 5.5 s where 40.0. The ready-made engine of this release is a new file,
strata-poly-windows-x64-b2b4ecb4224540ea.zip (RTX 20 to RTX 50, CUDA 13.0).

• --conversation-disk DIR (step 17 of docs/OPTIMIZATION_LOG.md): after every answer the engine writes what its
  conversation cache holds to DIR in the background, up to --conversation-disk-mib (default 4096): a conversation of
  the whole 144K context takes about 2.0 GiB, a 40K one 0.81 GiB. A file that is not whole is dropped at the next start,
  and the files are only taken up by the same model file, engine and options. The answers are written as fast as
  without it, and the tokens after a restart are those without one. It costs the RAM of the conversation in the engine
  as soon as it has answered (0.44 GiB at 40K, 0.78 GiB at 70K), inside the conversation cache's budget.
• Setup writes --conversation-disk <the data folder>\conversations --conversation-disk-mib 4096 on NVIDIA where the
  conversation cache is on and the data folder's disk has 20 GB free. It needs this release's engine: with an older one
  setup leaves the option out and says so. An install updated with UPDATE.bat keeps its config: add
  "--conversation-disk", "C:\\path\\to\\PolyStrata-data\\conversations" to the args of strata-<model>.json.
• Without --conversation-disk the engine writes v0.5.0's tokens.