Clang 18 for everyone and everything

[teh-cmc]

TODO: we need to patch loguru so that it passes clang18:

 FAILED: _deps/logurugitrepo-build/CMakeFiles/loguru.dir/loguru.cpp.o 
/Users/runner/work/rerun/rerun/.pixi/envs/cpp/bin/x86_64-apple-darwin13.4.0-clang++  -I/Users/runner/work/rerun/rerun/build/debug/_deps/logurugitrepo-src -march=core2 -mtune=haswell -mssse3 -ftree-vectorize -fPIC -fstack-protector-strong -O2 -pipe -stdlib=libc++ -fvisibility-inlines-hidden -fmessage-length=0 -isystem /Users/runner/work/rerun/rerun/.pixi/envs/cpp/include -g -isysroot /Applications/Xcode_16.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -Werror -MD -MT _deps/logurugitrepo-build/CMakeFiles/loguru.dir/loguru.cpp.o -MF _deps/logurugitrepo-build/CMakeFiles/loguru.dir/loguru.cpp.o.d -o _deps/logurugitrepo-build/CMakeFiles/loguru.dir/loguru.cpp.o -c /Users/runner/work/rerun/rerun/build/debug/_deps/logurugitrepo-src/loguru.cpp
/Users/runner/work/rerun/rerun/build/debug/_deps/logurugitrepo-src/loguru.cpp:1328:4: error: 'snprintf' will always be truncated; specified size is 5, but format string expands to at least 6 [-Werror,-Wformat-truncation]
 1328 |                         snprintf(level_buff, sizeof(level_buff) - 1, "% 4d", static_cast<int8_t>(verbosity));
      |                         ^
1 error generated.

---

Switch all pixi things to clang 18, which I've confirmed is capable of linking latest mimalloc on all platforms (as opposed to the current clang 16 we're using).

• Should also fix Web build fails on macOS ("LLVM error: section too large") #9849

[github-actions]

Latest documentation preview deployed successfully.

Result	Commit	Link
✅	c680fdd	https://landing-ja4x6c5q4-rerun.vercel.app/docs

^{Note: This comment is updated whenever you push a commit.}

[github-actions]

Web viewer built successfully. If applicable, you should also test it:

• I have tested the web viewer

Result	Commit	Link	Manifest
✅	c680fdd	https://rerun.io/viewer/pr/10110	+nightly +main

^{Note: This comment is updated whenever you push a commit.}

[teh-cmc]

Will undraft once (if) we pass full CI.

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/15387766380

[Wumpf]

note that on MacOS we still have to use system-clang for some thing - very annoying but couldn't figure out how to get things to link otherwise, see #10214

[Wumpf]

happy to take this over when I have a minute