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/nodes/textv3.rs

@@ -760,12 +760,12 @@ impl TextGeneratorV3 {
 				text: row
 					.get(PLAIN_TEXT_INPUT)
 					.map(to_text)
-					.unwrap_or_else(String::new),
+					.unwrap_or_default(),
 				mode: TextLayoutMode::PlainText,
 				font_family: row
 					.get(FONT_FAMILY_INPUT)
 					.map(to_text)
-					.unwrap_or_else(String::new),
+					.unwrap_or_default(),
 				font_size_pt: row
 					.get(FONT_SIZE_INPUT)
 					.map(|v| v.to_double())
@@ -776,7 +776,7 @@ impl TextGeneratorV3 {
 			}
 		} else {
 			TextLayoutRequest {
-				text: row.get(TEXT_INPUT).map(to_text).unwrap_or_else(String::new),
+				text: row.get(TEXT_INPUT).map(to_text).unwrap_or_default(),
 				mode: TextLayoutMode::OliveHtml,
 				font_family: String::new(),
 				font_size_pt: 0.0,
@@ -2257,7 +2257,7 @@ mod tests {
 		else {
 			panic!("expected a CPU coverage frame");
 		};
-		let stride = frame.linesize_bytes() as usize;
+		let stride = frame.linesize_bytes();
 		let at = y * stride + x * 16;
 		let mut out = [0f32; 4];
 		for (c, v) in out.iter_mut().enumerate() {