Tag three requirements the code already meets

R5 says in its own note that zoom_resolution.rs establishes it as a
pixel equality; that file was tagged FR-DSP-5 alone. FR-DEV-19's three
sub-clauses carry eighty-three tags between them while the parent had
none; MaskLayer, which is the thing they edit, now carries it. And
NFR-R3 — a crash in decode does not take down the application, the
image is marked failed — is exactly what the decoder's panic guard and
the face sweep's unreadable mark do, tagged FR-RAW-4 and NFR-SEC-1 and
not the clause that asked for them.

core/dr-decode/src/error.rs

@@ -27,7 +27,7 @@ pub enum DecodeError {
 
 /// Run a decoder call, and return a panic inside it as an error.
 ///
-/// TRACES: FR-RAW-4 | NFR-SEC-1
+/// TRACES: FR-RAW-4 | NFR-SEC-1 | NFR-R3
 /// rawler `panic!`s on some malformed input rather than returning `Err` — a
 /// DNG whose IFD claims a >50000 px image, for one, which is in the reference
 /// library. A panic on a worker thread ends the thread: the face sweep that