[build] Fix clangd under macOS, speed up CI slightly

[fhanau]

• Xcode 26.4 bumped the AppleClang version, add the new path for that
• Increase number of concurrent jobs to speed up downloading/fetching from cache now that we have larger runners
• Add new clang-tidy-only action, we may use this in the lint CI job in the future (would have to make sure header parsing and Rust linting are still active though)

[ask-bonk]

Let me review the changes carefully:

1. build/ci.bazelrc: Jobs bumped from 32 to 64, comment updated to reflect larger runners. This is a CI config tweak -- straightforward and low risk.

2. build/tools/clang_tidy/clang_tidy.bazelrc: New clang-tidy-only config that uses --output_groups=clang_tidy_checks (without the + prefix). The + in the existing clang-tidy config means "add to existing output groups", while omitting it means "only this output group." This is exactly the intended semantics for a "tidy-only" action that skips compilation. Correct.

3. build/tools/clang_tidy/clang_tidy.bzl: Adds the clang/21 include path for Xcode 26.4 before the existing clang/17 path. Order is correct -- newer version first so it takes precedence when present.

4. compile_flags.txt: Same clang/21 include path added for clangd, again before the clang/17 path. Consistent with the .bzl change.

All changes are build/tooling configuration, no runtime code, no behavioral changes, no backward compatibility concerns, no security implications. The logic is correct and consistent across the files.

LGTM

github run