Give the size of libvips' temporary file in "Running under upaas"

While pixa processes a large source, libvips writes its decoded pixels
to a file in /tmp, outside /var/lib/pixa and cache_max_bytes, and not
in the 1.6 GiB of resident memory. Measured on pixad for 8192x8192
sources, it is 192 MiB for an 8-bit RGB JPEG, 512 MiB for 16-bit RGBA,
and 1 GiB for a floating-point RGBA JPEG XL. The section now says so,
and that the file is memory where /tmp is a tmpfs.

Model: opus-5-5

TODO.md

@@ -72,7 +72,10 @@ P2: security: per-origin rate limiting
   `imageprocessor.New`; `PIXA_MAX_CONCURRENT_PROCESSING` still overrides it.
   "Running under upaas" in `README.md` gives the worst-case memory, measured,
   and tells the operator to set `PIXA_MAX_CONCURRENT_PROCESSING` and
-  `PIXA_UPSTREAM_CONNECTIONS` to fit the container's memory.
+  `PIXA_UPSTREAM_CONNECTIONS` to fit the container's memory. It also gives the
+  file libvips writes to `/tmp` for each large source, up to 1 GiB, measured,
+  which is outside `/var/lib/pixa` and `cache_max_bytes` and is memory where
+  `/tmp` is a tmpfs.
 - 2026-10-15 expired negative cache rows are removed on a schedule (closes
   #244): the row of a failed fetch, which makes pixa answer 502 for that source
   URL for 5 minutes, was deleted only when the same URL was requested again