* fix(nix): gate matrix extra to Linux in [all] profile
matrix-nio[e2e] depends on python-olm which is upstream-broken on modern
macOS (Clang 21+, archived libolm). Previously the [matrix] extra was
completely excluded from [all], meaning NixOS users (who install via [all])
had no Matrix support at all.
Add a sys_platform == 'linux' marker so [all] pulls in [matrix] on Linux
(where python-olm builds fine) while still skipping it on macOS. This
fixes the NixOS setup path without breaking macOS installs.
Update the regression test to verify the Linux-gated marker is present
rather than just checking matrix is absent from [all].
Fixes#4594
* chore: regenerate uv.lock with matrix-on-linux in [all]