Add tests, fix offset logic, and bump dependencies

- Introduce a comprehensive test suite (cli, app, board, tasks, audio)
  and a
  new TESTING.md document describing test execution.
- Correct vertical offset calculation in `BoardState` to use saturating
  subtraction and ceiling division.
- Clamp initial `Offset` values to the provided maximum in
  `Offset::new`.
- Export `load_audio` from the audio thread module and switch to
  `LoopedDecoder` for proper OGG looping.
- Update several crate versions: clap, ratatui, log, rodio, and
  bitflags.

Cargo.toml

@@ -10,9 +10,9 @@ opt-level = 3
 strip = true
 
 [dependencies]
-clap = { version = "4.5.53", features = ["derive"] }
-ratatui = "0.30.0"
-log = "0.4.29"
+clap = { version = "4.6.7", features = ["derive"] }
+ratatui = "0.30.2"
+log = "0.4.34"
 simplelog = "0.12.2"
-rodio = "0.22.2"
+rodio = "0.23.0"
 once_cell = "1.21.4"