clippy: clear the workspace errors and apply the machine fixes

- Mark the raw-pointer interop entry points unsafe with # Safety docs
  (oak-core upload/download/frame-from-pixels, oak-audio convert) and
  satisfy the existing callers (tests).
- mut_from_ref: allow with the ABI contract documented (the handle
  get_mut helpers in oak-timeline/oak-render/oak-task take the shared
  reference the C ABI passes; exclusivity is the caller's unsafe
  contract).
- Fix the eq_op in the white-balance normalization (green / green).
- Apply cargo clippy --fix across the workspace (redundant closures and
  field names, field reassignment, items after test modules, ...).
- Revert the replace_box fix in image_effect's clip_define: a
  redefinition must allocate a new box, otherwise the old clip handle
  stays valid and the HS-map replace contract (clip != clip2) breaks.
- 283 warnings remain; they are all non-machine-applicable
  (chunks_exact -> as_chunks needs a manual iter_mut, too_many_arguments,
  complex types, missing Safety docs, ...) and are tracked as the
  follow-up.

crates/oak-codec/src/proxymanager.rs

@@ -317,11 +317,10 @@ impl ProxyManager {
 	/// Locate an ffmpeg executable (empty string when none found).
 	pub fn find_ffmpeg(configured_path: &str) -> String {
 		// An explicitly configured path takes precedence if it is usable.
-		if !configured_path.is_empty() {
-			if is_executable_file(Path::new(configured_path)) {
+		if !configured_path.is_empty()
+			&& is_executable_file(Path::new(configured_path)) {
 				return absolute(configured_path);
 			}
-		}
 
 		// Fall back to searching the system PATH (split per platform:
 		// `;` on Windows, `:` elsewhere — see [`split_path_env`]).