This wasn't working before but it just requires a restart of the machine
for the changes to take effect. The namespace runners have this prebuilt
so this should work now.

The other workaround was flaky for unknown reasons so I'd prefer to go
back to this.

This updates our build script and CI to support Xcode 26 and macOS
Tahoe. **This doesn't update the Ghostty app to resolve any Tahoe
issues.**

For CI, we've added a new build job that runs on macOS Tahoe with Xcode
26. I've stopped short of updating our tip release job, since I think I
want to wait until I verify a bit more about Tahoe before we flip that
bit. Also, ideally, we'd run Xcode 26 on Sequoia (macOS 15) for
stability reasons and Namespace doesn't have Xcode 26 on 15 yet.

For builds, this updates our build script to find Metal binaries using
`xcodebuild -find-executable` instead of `xcrun`. The latter doesn't
work with Xcode 26, but the former does and also still works with older
Xcodes. I'm not sure if this is a bug but I did report it: FB17874042.

The major idea behind the refactor is to split the `build.zig` file up into
distinct `src/build/*.zig` files. By doing so, we can improve readability of
the primary `build.zig` while also enabling better reuse of steps. Our
`build.zig` is now less than 150 lines of code (of course, it calls into a lot
more lines but they're neatly organized now).

Improvements:

  * `build.zig` is less than 150 lines of readable code.
  * Help strings and unicode table generators are only run once when multiple
    artifacts are built since the results are the same regardless of target.
  * Metal lib is only built once per architecture (rather than once per artifact)
  * Resources (shell integration, terminfo, etc.) and docs are only
    built/installed for artifacts that need them

Breaking changes:

  * Removed broken wasm build (@gabydd will re-add)
  * Removed conformance files, shell scripts are better and we don't run
    these anymore
  * Removed macOS app bundle creation, we don't use this anymore since we
    use Xcode

## Some History

Our `build.zig` hasn't been significantly refactored since the project started,
when Zig was _version 0.10_. Since then, the build system has changed
significantly. We've only ever duct taped the `build.zig` as we needed to
support new Zig versions, new features, etc. It was a mess.

The major improvement is adapting the entire Ghostty `build.zig` to the Step
and LazyPath changes introduced way back in Zig 0.12. This lets us better take
advantage of parallelism and the dependency graph so that steps are only
executed as they're needed.

As such, you can see in the build.zig that we initialize a lot of things, but
unless a final target (i.e. install, run) references those steps, _they'll
never be executed_. This lets us clean up a lot.

This reverts commit aad101565abff3368af76e8c2e93cbe7c278a89f.

This enables the compile options and Xcode configuration so that logging
in Metal shaders shows up in our Xcode debug console. This doesn't add
any log messages, but makes it so that when we iterate on the shaders in
the future, we can add and see logs to help us out.