Linking: Remove link_SDL2* methods, update README, and fix race condition

[mjakeman]

I've split this out from #25 to compare/contrast the approach.

Changes:

• Fix race condition in build.zig: The run step did not depend on install step, causing the tests to be run from zig-cache instead of zig-out. That meant that even with a correct rpath, it would still fail.
• Remove link_SDL2* methods: The methods cause a compile error when used directing the user to the readme.
• README Changes: Contains instructions on how to properly link SDL, and moves the RPath parts to the prebuilt section (where it's particularly relevant, as system libraries/frameworks will be installed in global search paths).

When using any of the old link_SDL2* methods, the user will see this:

To Do:

• Test this with a brand new repo

[mjakeman]

Race condition seems like the same issue as: ziglang/zig#22117

[Copilot]

Pull Request Overview

This PR removes deprecated SDL linking methods and fixes a race condition in the test execution. The main purpose is to improve the build system reliability and guide users toward proper SDL linking practices.

• Replaces deprecated link_SDL2* methods with compile errors directing users to the README
• Fixes race condition by ensuring test runs depend on install step
• Updates README with comprehensive SDL linking instructions and prebuilt library setup

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
build.zig	Removes public linking methods, adds compile errors for deprecated functions, fixes test dependency race condition
README.md	Adds detailed SDL linking examples and reorganizes prebuilt library documentation

[hazeycode]

Thanks!