[devmobasa]

Summary

This PR adds three CI changes, one commit each:

• Build the Nix flake. The "Nix flake" job only ran nix build --dry-run,
  which evaluates the packages but never builds them. [BUG] Installing via nix flake fails #432 shipped because of
  this: the check phase failed only during a real build.
• Check the minimum Rust version. Both manifests declare
  rust-version = "1.99.0" so nixpkgs can build Wayscriber. Every job used
  the 1.99.1 development toolchain, so nothing tested that floor.
• Pin the GTK widget job to ubuntu-24.04. ubuntu-latest is moving to a
  newer image, and with newer GTK and Weston the configurator contract below
  fails.

Changes

ci: build the Nix flake instead of only evaluating it

• Adds check nix-build. It builds .#wayscriber, .#wayscriber-configurator,
  and the x86_64-linux development shell, including each package's check
  phase, and prints the build logs. It uses the same list of flake outputs as
  check nix-instantiation.
• The "Nix flake" job runs it after the existing checks. Its timeout rises
  from 20 to 45 minutes for the release builds. The check name stays the same.

ci: check the workspace with the minimum Rust version

• Adds ci minimum-rust and a new "Minimum Rust" job that runs it. The
  command:
  • reads rust-version from both manifests, which must agree
  • installs that toolchain with rustup, using the minimal profile and
    --no-self-update
  • runs cargo +<version> check --locked --workspace --all-targets with
    --all-features and with --no-default-features
• The explicit +<version> is needed because rust-toolchain.toml selects
  1.99.1 inside the checkout.
• version check also requires both crates to declare the same
  rust-version, and rust-toolchain.toml to pin a channel no older than
  it.
• The version-bump test fixture now writes the real rust-version into its
  synthetic manifests.

ci: pin the GTK widget job to Ubuntu 24.04

app::component::tests::language_changes_rebuild_the_window_in_place fails
with GTK 4.24 and Weston 15. Shortcut recording ends before the first
language switch. Giving Weston a fake seat (--fake-seat) removes the seat
warning, but the test still fails, so a missing seat is not the cause. The
job passes on Ubuntu 24.04. The pin keeps a runner image change from
breaking this required check until the contract is investigated separately.

Validation

• The C# tool tests pass, including new tests that record the exact commands
  check nix-instantiation, check nix-build, and ci minimum-rust would
  run, and new version check cases for both Rust rules.
• dotnet format passes, and so do version check, assets check,
  check nixpkgs-recipe, check legacy-tools, and
  check rust-source-coverage.
• ci minimum-rust was run locally. It installs and selects 1.99.0, and
  both feature sets check cleanly.
• Each commit builds and passes the C# tests on its own.
• The new "Nix flake" build step and the "Minimum Rust" job run for the first
  time on this PR.

Notes

• "Minimum Rust" is a new check name. Add it to branch protection if it
  should be required.
• The flake also advertises aarch64-linux, which still has no build job. An
  Arm runner lane is a possible follow-up.