chore: upgrade to TeX Live 2026

TeX Live 2026 was released on 2026-03-01. When a new release comes out,
the previous version is archived to historic mirrors (tug.org/historic),
which have very limited availability. This caused CI to fail immediately
with "No mirror available" for texlive_version: 2025.

Upgrading to 2026 which is now propagating to active CTAN mirrors.

Ref: https://tug.org/pipermail/tex-live/2026-March/052232.html

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: kim-em

.github/workflows/ci.yml

@@ -67,7 +67,7 @@ jobs:
             - name: Install PDF Dependencies
               uses: zauguin/install-texlive@v4
               with:
-                  texlive_version: 2025
+                  texlive_version: 2026
                   packages: |
                       scheme-minimal
                       l3packages