#1615: Auto caps CPU frames in flight from free memory after a job switches to the CPU (#3458)

A GPU project whose Render Queue job measured the CPU faster (#3335) rendered
every frame of a batch at once on the CPU, one per thread, with no memory cap:
on a 16-thread machine at about 1.4 GB a frame the app ran out of RAM and
exited without an error (0.23.1, Windows).

While a job renders on the CPU after a GPU frame measured its working memory,
CPU frames in flight now fit half the memory the system has free at that size
per frame, 1 to one per thread, settled once per job and logged. The probe's
timed CPU frames use the same cap. EFFECTCRAFT_CPU_INFLIGHT still overrides
it; Software Rendering projects, with no measurement, are unchanged. The host
reads the free memory through the new JobOptions::free_memory.

docs/preferences.md

@@ -100,10 +100,13 @@ Commands (CLI, MCP, control channel):
   one; `EFFECTCRAFT_GPU_MEMCAP_MB` sets it), keeping room for the upload cache to fill. It
   renders one frame at a time where the memory can't be measured (Metal, WebGPU, GL, or the
   cap turned off) and once frames have had to wait for memory. The render log names the count.
-  `EFFECTCRAFT_GPU_INFLIGHT` overrides it. CPU renders have no such setting: they keep one
-  frame per thread in flight, so their memory grows with the core count. Set
-  `EFFECTCRAFT_CPU_INFLIGHT` to a whole number of frames (at least 1) to cap the frames in
-  flight of every Render Queue render and export, `effectcraft-cli render` included
+  `EFFECTCRAFT_GPU_INFLIGHT` overrides it. CPU renders have no such setting. A job on GPU
+  Acceleration that renders on the CPU (it measured the CPU faster, or the GPU stopped) fits
+  its CPU frames in flight into half the memory the system has free, at the size a GPU frame
+  measured, from 1 to one per thread; the app log names the count. Other CPU renders (Software
+  Rendering projects) keep one frame per thread in flight, so their memory grows with the core
+  count. Set `EFFECTCRAFT_CPU_INFLIGHT` to a whole number of frames (at least 1) to cap the
+  frames in flight of every Render Queue render and export, `effectcraft-cli render` included
   ([#1615](https://github.com/storytold/effectcraft/issues/1615)); previews don't read it. One
   exception: while a job on GPU Acceleration times both compositors on its first frames, it
   renders them under the cap but keeps them all (up to 12) until it has chosen. That timing