Migrate to SDL3 (except for Android and Switch)

Switch desktop/emscripten builds from SDL2 to SDL3, applying only the
API changes that cannot compile through the SDL_oldnames.h aliases. Pure
renames will be swept in a follow-up commit.

.github/workflows/linux.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     strategy:
       matrix:
         build-type: ["Debug", "Release"]
@@ -17,7 +17,7 @@ jobs:
     - name: Install Deps
       run: |
         sudo apt update
-        sudo apt install libsdl2-dev libsdl2-ttf-dev librtmidi-dev nlohmann-json3-dev
+        sudo apt install libsdl3-dev libsdl3-ttf-dev librtmidi-dev nlohmann-json3-dev
 
     - name: Build
       run: |

.github/workflows/switch.yml

@@ -1,5 +1,7 @@
 name: Nintendo Switch Build
-on: [push, pull_request]
+# Disabled: devkitPro provides no SDL3 portlib yet, so the Switch build cannot be
+# migrated off SDL2. Run manually once an SDL3 portlib is available.
+on: workflow_dispatch
 
 jobs:
   build:

.github/workflows/windows.yml

@@ -8,9 +8,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - sys: mingw32
-            package: "system3-sdl2-32bit"
-            deps: ""
+          # MSYS2 provides no i686 (mingw32) SDL3 packages, so the 32-bit MSYS2
+          # build is dropped. 32-bit Windows is still covered by the MSVC Win32 build.
           - sys: ucrt64
             package: "system3-sdl2-64bit"
             deps: "nlohmann-json:p"
@@ -25,8 +24,8 @@ jobs:
       with:
         msystem: ${{ matrix.sys }}
         pacboy: >-
-          SDL2:p
-          SDL2_ttf:p
+          sdl3:p
+          sdl3-ttf:p
           rtmidi:p
           ${{ matrix.deps }}