core: merge oak-common into oak-core

oak-common is gone; its modules (configstore, xmlutils, ocioutils,
oiioutils, colormath, colortransform, videoparams, ffmpegutils, ...)
now live in oak-core alongside the value types. The render value/GPU
types moved too: backend (wgpu context + DisplayRenderer), color
(ColorProcessor over ocio-rs), texture, frame, and the commonutil
config helpers.

Fix-ups to make the merged tree build and pass tests:

- oak-core Cargo.toml: wgpu back to 25 (the moved backend code is
  written against that API generation); add the toml/quick-xml/image
  deps oak-common carried.
- lib.rs: drop the duplicate 'pub mod error;'.
- error.rs: unified OAKCORE_* codes; restore Error::new() and
  From<OcioError> from oak-common's error type.
- backend.rs/color.rs: oak_core::/oak_render:: self-references
  rewritten to crate::; the shaderfx-dependent GPU effect test moved
  to oak-render's shaderfx tests (shaderfx depends on oak-node and
  cannot live in oak-core).
- oak-render's error module re-exports oak_core::error::{Error,
  Result}; the OAKRENDER_* codes stay as the public-code contract.
- oak-node jobs.rs: ColorProcessor imported from oak_core::color.
- Integration tests repointed at oak_core::{texture, frame, backend,
  color, colormath}.
- the display-ICC regression test treats an empty OAK_DISPLAY_ICC as
  unset, matching displayicc::env_override_icc.

crates/oak-task/src/task.rs

@@ -21,7 +21,7 @@
 //! shared lifecycle lives here; the per-task work is supplied through
 //! [`TaskBehavior`] (a trait object, per architectural decision #1 in
 //! README.md). Cancellation rides on a shared
-//! `oak_common::cancelatom::CancelAtom` (single-lib unification: the old
+//! `oak_core::cancelatom::CancelAtom` (single-lib unification: the old
 //! oakrender cancelatom C ABI is gone).
 //!
 //! CPP-PARITY: src/task/src/task.h
@@ -39,7 +39,7 @@
 use std::sync::atomic::{AtomicI64, Ordering};
 use std::sync::{Arc, Condvar, Mutex};
 
-use oak_common::cancelatom::CancelAtom;
+use oak_core::cancelatom::CancelAtom;
 
 use crate::error::{Error, Result};