Improve GitHub CI and dependency maintenance

[beamiter]

What changed

• add a GitHub Actions CI workflow for Rust formatting, Clippy, unit tests, and an optimized release build
• install the Linux development libraries required by the Wayland/X11/egui/wgpu stack
• cache Cargo build data independently for lint, test, and release jobs
• add read-only workflow permissions, concurrency cancellation, timeouts, colored Cargo output, and backtraces
• use the current actions/checkout@v7 action
• add grouped Dependabot updates for Cargo dependencies and GitHub Actions
• commit Cargo.lock for reproducible application builds and stop ignoring it
• apply the formatting differences detected by the first CI run

Why

The repository did not have a GitHub Actions workflow, so regressions in formatting, lints, tests, or release linking could reach master without an automated check. The application also resolved dependencies without a committed lockfile. This PR establishes a Linux-focused, reproducible CI baseline matching the project's documented platform support.

Validation

The final GitHub Actions run passes all four jobs on Ubuntu 24.04:

• Rustfmt
• Clippy
• unit tests
• release build

Clippy currently reports the repository's existing lint backlog without promoting every warning to a blocking error. This keeps CI usable now while allowing lint strictness to be tightened incrementally in focused follow-up changes.