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-node/src/ops.rs

@@ -252,9 +252,7 @@ pub fn set_value_at_time_command(
 					.and_then(|e| e.core.keyframe_track(&input, element))
 					.and_then(|t| {
 						t.keys()
-							.iter()
-							.filter(|k| k.time <= time)
-							.next_back()
+							.iter().rfind(|k| k.time <= time)
 							.map(|k| k.interpolation)
 					})
 					.unwrap_or(Interpolation::Linear);