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-pipeline/src/mask.rs

@@ -1304,7 +1304,12 @@ impl PartialEq for MaskPart {
     }
 }
 
+/// TRACES: FR-DEV-19
 /// One local adjustment: a rule about *where*, plus a chain saying *what*.
+///
+/// The *where* is editable after the fact — composed from parts (FR-DEV-19a),
+/// painted into and out of (FR-DEV-19b), shown (FR-DEV-19c) — and every edit
+/// to it is geometry and parameters in this struct, never a raster in a file.
 pub struct MaskLayer {
     /// Stable identity, for the sidecar and for merge (FR-NC-9).
     pub id: String,