Adopt SDL3 default for Prime Hunters

[mstan]

Summary

• default the MPH launcher and build scripts to SDL3
• add explicit SDL2 fallback selection for launcher and runner builds
• package SDL3.dll by default on Windows, with SDL2 selected only when requested
• build pinned SDL3 in the Steam Deck/Linux packaging container while retaining SDL2 fallback support
• document MPH as the first SDL3 target integration

Depends on

• Make SDL3 the default runner frontend ndsrecomp#16

Validation

• configured and built SDL3 mph-recomp-ui
• configured and built SDL2 fallback mph-recomp-ui
• ran mph_mod_provider_test
• copied the local MPH ROM into the worktree and generated title banks
• built title-aware SDL3 runner against Make SDL3 the default runner frontend ndsrecomp#16 branch
• launched via staged SDL3 recomp-ui folder with patched runner
• user manually confirmed SDL3 passes, including bottom-screen touch
• git diff --check

Not committed

• private ROM, generated banks, build outputs, scratch launch folder, and validation performance logs remain untracked/ignored locally.