refactor: workspace layout — crates/, app at root, legacy C++ removed

Single mechanical restructure commit:
- root Cargo.toml = oakapp bin + workspace; one cargo build produces
  oakapp, oak-cli, oak-worker, liboakengine.dylib
- app/rust/src -> src/ (app at repo root, no rust/ nesting)
- src/<mod>/rust -> crates/oak<mod>; src/oakcore-rs -> crates/oakcore;
  src/bindings/oakotio -> crates/oakotio; src/engine/rust ->
  crates/oakengine (keeps cdylib+staticlib+rlib)
- public C headers include/<mod>/ -> crates/oakengine/include/<mod>/
- OFX SDK headers vendored into crates/oakplugin/ofx/ (HostSupport gone)
- legacy deleted: old src/ C++ modules, engine/, core/, ffmpeg_bridge/,
  app/ (Qt), cli/worker C++, root CMakeLists, third_party/KDDockWidgets
  submodule, otio-install, all build-* output (~40GB)
- oakstorage kept but excluded from the workspace (skeleton w/ todos);
  gpui excluded (own workspace)
- verified: cargo build green, cargo test --workspace 1845/0
  (with the documented OCIO_RS_* env override for the homebrew OCIO)

crates/oakcommon/src/error.rs

@@ -0,0 +1,154 @@
+// Oak Video Editor - Non-Linear Video Editor
+// Copyright (C) 2026 Oak Team
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Error codes, mirroring `include/common/error.h`; project-wide
+//! -MMCCCC scheme (module 01), pass-through untranslated.
+
+/// Success.
+pub const OAKCOMMON_OK: i32 = 0;
+/// Empty handle or invalid argument.
+pub const OAKCOMMON_E_INVALID: i32 = -10001;
+/// Call not valid in the current state.
+pub const OAKCOMMON_E_STATE: i32 = -10002;
+/// The underlying operation failed.
+pub const OAKCOMMON_E_FAILED: i32 = -10003;
+/// Index out of range / entry not found.
+pub const OAKCOMMON_E_NOT_FOUND: i32 = -10004;
+/// Allocation failed.
+pub const OAKCOMMON_E_NOMEM: i32 = -10005;
+
+/// Crate-internal result type.
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Crate-internal error.
+#[derive(Debug)]
+pub enum Error {
+	/// Empty handle or invalid argument.
+	Invalid,
+	/// Wrong state.
+	State,
+	/// Operation failed (context string is log-only).
+	Failed(String),
+	/// Not found.
+	NotFound,
+	/// Out of memory.
+	NoMem,
+}
+
+impl Error {
+	/// Map to the public error code.
+	pub fn code(&self) -> i32 {
+		match self {
+			Error::Invalid => OAKCOMMON_E_INVALID,
+			Error::State => OAKCOMMON_E_STATE,
+			Error::Failed(_) => OAKCOMMON_E_FAILED,
+			Error::NotFound => OAKCOMMON_E_NOT_FOUND,
+			Error::NoMem => OAKCOMMON_E_NOMEM,
+		}
+	}
+
+	/// Create a [`Error::Failed`] carrying a context message (log-only).
+	///
+	/// Convenience constructor used by the OCIO/`image` wrappers in
+	/// `ocioutils.rs` / `oiioutils.rs` when an underlying library call fails.
+	pub fn new(message: impl Into<String>) -> Self {
+		Error::Failed(message.into())
+	}
+}
+
+impl From<ocio_rs::OcioError> for Error {
+	fn from(e: ocio_rs::OcioError) -> Self {
+		// The Display impl of `OcioError` always produces a non-empty message
+		// (every variant carries text or a fixed phrase); it becomes the
+		// log-only context of `Error::Failed`.
+		Error::Failed(e.to_string())
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn public_codes_match_header_values() {
+		// Load-bearing values from include/common/error.h (module 01).
+		assert_eq!(OAKCOMMON_OK, 0);
+		assert_eq!(OAKCOMMON_E_INVALID, -10001);
+		assert_eq!(OAKCOMMON_E_STATE, -10002);
+		assert_eq!(OAKCOMMON_E_FAILED, -10003);
+		assert_eq!(OAKCOMMON_E_NOT_FOUND, -10004);
+		assert_eq!(OAKCOMMON_E_NOMEM, -10005);
+	}
+
+	#[test]
+	fn error_code_maps_each_variant() {
+		assert_eq!(Error::Invalid.code(), OAKCOMMON_E_INVALID);
+		assert_eq!(Error::State.code(), OAKCOMMON_E_STATE);
+		assert_eq!(Error::Failed("boom".to_string()).code(), OAKCOMMON_E_FAILED);
+		assert_eq!(Error::NotFound.code(), OAKCOMMON_E_NOT_FOUND);
+		assert_eq!(Error::NoMem.code(), OAKCOMMON_E_NOMEM);
+	}
+
+	#[test]
+	fn error_codes_are_all_distinct() {
+		let codes = [
+			Error::Invalid.code(),
+			Error::State.code(),
+			Error::Failed(String::new()).code(),
+			Error::NotFound.code(),
+			Error::NoMem.code(),
+		];
+		for (i, a) in codes.iter().enumerate() {
+			for b in &codes[i + 1..] {
+				assert_ne!(a, b);
+			}
+			// Errors are strictly negative; OK stays zero.
+			assert!(*a < 0);
+		}
+	}
+
+	#[test]
+	fn failed_message_is_preserved_in_debug() {
+		// The context string is log-only but must survive to the log.
+		let e = Error::Failed("context info".to_string());
+		let dbg = format!("{e:?}");
+		assert!(dbg.contains("context info"));
+	}
+
+	#[test]
+	fn result_alias_round_trips_ok_and_err() {
+		let ok: Result<i32> = Ok(7);
+		assert_eq!(ok.unwrap(), 7);
+		let err: Result<i32> = Err(Error::NotFound);
+		assert_eq!(err.unwrap_err().code(), OAKCOMMON_E_NOT_FOUND);
+	}
+
+	#[test]
+	fn new_creates_failed_with_message() {
+		let e = Error::new("context info");
+		assert!(matches!(e, Error::Failed(_)));
+		assert_eq!(e.code(), OAKCOMMON_E_FAILED);
+		assert!(format!("{e:?}").contains("context info"));
+	}
+
+	#[test]
+	fn ocio_error_converts_to_failed() {
+		let e = Error::from(ocio_rs::OcioError::InvalidInput("bad colorspace".to_string()));
+		assert!(matches!(e, Error::Failed(_)));
+		assert_eq!(e.code(), OAKCOMMON_E_FAILED);
+		assert!(format!("{e:?}").contains("bad colorspace"));
+	}
+}