WIP: Fix all warnings

[Rainyan]

Description

• Fix all compiler warnings for MSVC and the compilers in Steam Runtime SDK versions 3 and 4 (most importantly the ones used by the CI)
• Turns warnings into errors

This is a refactor in spirit, but there have technically been some changes where a bug was discovered and fixed, and/or undefined behaviour was tamed. In practice though, there should be no functional changes introduced.

Current WIP:

• Successful Clang 21.1.0 run
• Commit cleanup
• Update docs
• Verify all of the listed compilers once more after rebase to master (probably unrealistic to maintain all of them passing all the time, but just to give us a good baseline point in git history where that was the case)

Toolchain

The following compilers are documented as "supported" in CONTRIBUTING.md by this PR, and they've been confirmed to not emit any warnings in release nor debug presets:

• Windows MSVC v143 - VS 2022 C++ x64/x86 build tools (Latest)
• Windows MSVC version used by the latest windows-2025 CI runner image (Microsoft.VisualStudio.Component.VC.Tools.x86.x64)
• Linux GCC 10 steamrt3 'sniper' (currently used for Linux CI builds)
• Linux GCC 14 steamrt3 'sniper'
• Linux Clang 11 steamrt3 'sniper' wontfix: compile issues with c++20 stdlib includes
• Linux GCC 14 steamrt4
• Linux Clang 19 steamrt4

I decided to drop support for the problematic Clang compiler in the steamrt3 container, because it's over 5 years old and unlikely to get patches or backports anymore, and because we do not currently use it for the CI anyway. If you want to use Clang, the steamrt4 option is the recommended one (alternatively see the "Other compilers" section of this message below, although beware those are unsupported.

Of the list above, the compilers currently used by the GitHub CI builds are:

• The windows-2025 runner image version of MSVC for Windows
• The GCC 10 steamrt3 inside the 'sniper' container for Linux

While we don't use steamrt4 yet, support for it was included to make the migration smoother in the future, either to GCC 14 or Clang 19.

The Windows runner image is not currently pinnable, so sporadic breakage is possible. The Linux steamrt container could be pinned; we're currently using sniper/sdk:latest but as it seems rather stable, pinning is probably not necessary for now. Some safeguards have been implemented by this PR as documented, such as disabling warnings-as-errors for tagged release builds, to minimize breakage from unexpected build environment changes.

Other compilers

Additionally, the following compilers were verified to not emit any warnings, although not going to "officially" maintain support for them (nor any other compilers outside the ones listed in "Toolchain"):

• Linux GCC 11.4.0-1ubuntu1~22.04.2
• Linux Clang 14.0.0-1ubuntu1.1
• Linux GCC 16-20251221 Snapshot (latest as of 2025/12/28)
• TODO Linux Clang 21.1.0 (latest as of 2025/12/28)

Linked Issues