Upgrade Odin version

[glennj]

Files affected

• README.md
• ODIN_VERSION
• bin/fetch-ols-odinfmt.sh
• docs/INSTALLATION.md
• .github/workflows/pr.yml
• .github/workflows/test-all.yml

And the test runner's Dockerfile (take note of this comment in it)

[rmonnet]

It looks like the latest version is dev-2026-01 but the laytan/setup-odin used in the github workflow only support 2025-12. I need to check which one since there are 2 december releases. 2025-12a and 2025-12 so we can get a consistent version across the track and the test runner.

Or we could wait for laytan/setup-odin to support 2026-01 and jump to that.

@glennj, did you have a specific problem or just wrote the issue so we don't forget to update?

[glennj]

no problem.