refactor(crates): implement std::error::Error for all module error enums

thiserror derive across oakundo/oakcommon/oaknode/oaktimeline/oaktask/
oakotio/oakcodec/oakaudio/oakrender/oakplugin/oakengine/oakstorage;
Display carries the module prefix and the Failed context, source()
stays default except oakotio's #[from] forwarding. code() mappings and
variants unchanged; each error.rs gains Display/object-safety/code
regression tests.

crates/oakcodec/src/error.rs

@@ -39,19 +39,25 @@ pub const OAKCODEC_ABI_VERSION: u32 = 1;
 pub type Result<T> = std::result::Result<T, Error>;
 
 /// Crate-internal error.
-#[derive(Debug)]
+#[derive(Debug, thiserror::Error)]
 pub enum Error {
 	/// Null handle or invalid argument.
+	#[error("codec: invalid argument")]
 	Invalid,
 	/// Wrong state.
+	#[error("codec: invalid state")]
 	State,
 	/// Operation failed (context string is log-only).
+	#[error("codec: operation failed: {0}")]
 	Failed(String),
 	/// Not found.
+	#[error("codec: not found")]
 	NotFound,
 	/// Out of memory.
+	#[error("codec: out of memory")]
 	NoMem,
 	/// The operation was cancelled.
+	#[error("codec: operation cancelled")]
 	Cancelled,
 }
 
@@ -68,3 +74,60 @@ impl Error {
 		}
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	/// Every variant must produce a non-empty `Display` message; the
+	/// `Failed` variant must surface its context string.
+	#[test]
+	fn display_is_non_empty() {
+		for msg in [
+			Error::Invalid.to_string(),
+			Error::State.to_string(),
+			Error::Failed("context".into()).to_string(),
+			Error::NotFound.to_string(),
+			Error::NoMem.to_string(),
+			Error::Cancelled.to_string(),
+		] {
+			assert!(!msg.trim().is_empty(), "empty Display message");
+		}
+		assert!(
+			Error::Failed("context".into())
+				.to_string()
+				.contains("context")
+		);
+	}
+
+	/// `Error` must be usable behind a trait object.
+	#[test]
+	fn error_is_object_safe() {
+		let errs: Vec<Box<dyn std::error::Error>> = vec![
+			Box::new(Error::Invalid),
+			Box::new(Error::State),
+			Box::new(Error::Failed("context".into())),
+			Box::new(Error::NotFound),
+			Box::new(Error::NoMem),
+			Box::new(Error::Cancelled),
+		];
+		assert_eq!(errs.len(), 6);
+	}
+
+	/// No variant wraps a downstream error, so `source()` stays `None`.
+	#[test]
+	fn source_is_none() {
+		assert!(std::error::Error::source(&Error::Failed("context".into())).is_none());
+	}
+
+	/// The `code()` mapping must be unchanged by the `Error` trait impl.
+	#[test]
+	fn code_mapping_unchanged() {
+		assert_eq!(Error::Invalid.code(), OAKCODEC_E_INVALID);
+		assert_eq!(Error::State.code(), OAKCODEC_E_STATE);
+		assert_eq!(Error::Failed("context".into()).code(), OAKCODEC_E_FAILED);
+		assert_eq!(Error::NotFound.code(), OAKCODEC_E_NOT_FOUND);
+		assert_eq!(Error::NoMem.code(), OAKCODEC_E_NOMEM);
+		assert_eq!(Error::Cancelled.code(), OAKCODEC_E_CANCELLED);
+	}
+}