[lianxiaozhi]

Fixes #10452.

QOI_OP_RUN skips the shared pixel-index update. An initial run of opaque black followed by a later INDEX reference can therefore decode as transparent black. This applies the fix suggested by @manit2004 in the report.

Changes proposed in this pull request:

• Update the previously seen pixel index in the RUN branch, before discarding alpha for RGB output.
• Add six regression cases covering RGB/RGBA and run lengths 1, 2 and 62. The three RGBA cases fail before the fix; the existing first-chunk INDEX regression remains passing.
• Add a 13.1.0 release note.

Validation on macOS arm64 with Python 3.11.15, using a full editable source build of Pillow 13.1.0.dev0 and its native extensions:

• Self-test: 59 passed. QOI tests: 14 passed.
• Test suite: 5,587 passed, 372 skipped, 3 expected failures. The screen-capture test was explicitly deselected because no display is available; it also fails on unchanged upstream code. The skips are for platform-specific tests, optional dependencies and extra image fixtures.
• tox -e lint and tox -e mypy: passed.
• All eight official QOI test images match their PNG references exactly, including edgecase.qoi, which failed before the fix.

[lianxiaozhi]

Closing in favor of #10453, which was opened first and contains the same decoder fix. Thanks @manit2004 for the report and fix.

The additional regression coverage and release note are available separately in lianxiaozhi@0078905, based on the author's PR head. This commit changes only the tests and release note, so it can be reused without duplicating the decoder change. The local validation results are recorded in this PR's description and in the follow-up on #10452.