release: nifkit v0.3.1

[puffball1567]

Release scope

• make omitted CodecLimits unbounded at the application-policy level; platform resource limits and int width still apply
• require network-facing integrations to choose and pass explicit limits for their own trust boundary
• add Linux/macOS/Windows AddressSanitizer coverage for ARC and ORC
• retain Linux Valgrind, LeakSanitizer, UBSan, and TSan coverage where supported
• skip CI for Markdown/docs-only changes while retaining it for source, test, configuration, and workflow changes
• update the package version to 0.3.1

Verification

• PR ci: add ASan coverage for ARC and ORC #36: Linux/macOS/Windows verification and sanitizer coverage passed
• PR fix: leave omitted codec limits unbounded #39: Linux/macOS/Windows verification, ASan, supported UBSan/TSan/LSan, and ARC/ORC Valgrind passed
• local: nimble verify -y

After this release PR is merged, create the annotated v0.3.1 tag from main and publish the corresponding GitHub Release.