vc_render_tifxyz: warn when --cache-gb is not below the memory available to the process

[nerln]

Refs #1404 (the --cache-gb half). Prior work: #1415 addressed both halves of the issue (a clamp plus a torn-TIFF check) and was closed by the repository time limit without a human review; the torn-TIFF half is now in #1695. This PR only adds the warning the issue asks for on the --cache-gb side. Nothing is clamped and no default changes.

--cache-gb (default 16) at or above the memory the process can use made the render sit at the memory ceiling with no output and no log line, indistinguishable from a slow render (the report in #1404 is a Docker host with a 15 GB limit). The tool now prints one line to stderr when the requested cache is not below the usable memory: the cgroup limit when one is set (memory.max or memory.limit_in_bytes), otherwise physical RAM. On Windows there is no check.

Reproduced on main (1f544fa71) with a 3-slice render of a PHerc0125 segment, on a 16 GB machine:

$ vc_render_tifxyz ... --cache-gb 999          # before: renders, no warning
$ vc_render_tifxyz ... --cache-gb 999          # after
Warning: --cache-gb 999 is not below the memory available to this process (16.0 GB): the render can stall without any output. Lower --cache-gb.

@hendrikschilling, you merged #1528 on this file: could you take a look when you have a moment?

The patch was prepared with Claude Code under my direction; I built it and ran the reproduction above locally.

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