feat: allow freeing of ustring table for debugging cases

[lgritz]

This patch optionally frees the contents of the ustring table as the process is exiting. It's triggered one of two ways:

• OIIO::attribute("ustring:cleanup", 1);
• Environment variable OIIO_USTRING_CLEANUP=1

If neither is used, the teardown won't happen, thus avoiding any pointless work when it should be exiting the process.

The purpose of this is that when using certain debugging tools, the ustring table (which only grows and never frees) looks like a memory leak, even though it's by design. Someimes, it's useful to make it not appear ot leak, and you are happy for a large table to spend time pointlessly freeing all the data.

Fixes #3913

[lgritz]

Another independent stab that is an alternate to #5193. This PR still establishes the same external API for requesting that the ustring table be cleaned up, but I change the absolute minimal amount of the existing code.

[Copilot]

Pull request overview

This PR adds an opt-in shutdown-time cleanup path for the global ustring table so that leak-detection tools (e.g. valgrind) don’t report the intentionally-retained ustring storage as leaks. The cleanup is enabled either via OIIO::attribute("ustring:cleanup", 1) or the OIIO_USTRING_CLEANUP environment variable.

Changes:

• Introduce a global oiio_ustring_cleanup flag (env-var initialized) and add optional teardown logic to ustring table destruction.
• Wire the new "ustring:cleanup" option into OIIO::attribute() / OIIO::getattribute().
• Document the new attribute in the public API header.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/libutil/ustring.cpp	Adds the cleanup flag and implements optional destruction/freeing of table entries and pools at exit.
src/libOpenImageIO/imageio.cpp	Exposes "ustring:cleanup" through global attribute/getattribute plumbing.
src/include/OpenImageIO/imageio.h	Documents the new global attribute.
src/include/imageio_pvt.h	Declares the exported cleanup flag for cross-library access.

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[lgritz]

@fpsunflower Is this one more to your liking? It's more of a truly minimal modification to the existing ustring.cpp code.

And @jreichel-nvidia, so sorry, but could you also try this one and verify that it squashes all the detectable leaks?