Let a mask cover a whole category, not just one instance

`MaskSource` could say "instance 3 of that segmentation run" but had no way
to say "the sky". Adding `Category { signature, name }` beside `Subject` is
what lets a local adjustment attach to a semantic category at all.

Stored as identity like a subject, and for the same reason: the coverage is
megabytes and is reproducible by running the same model over the same image,
so the sidecar carries what finds it again and the session carries pixels.

## A name rather than an index

An index would be smaller and would match `Subject`. It would also be a bug.
The grouping lives in `models/scene/categories.txt`, which is editable by
design — adding one category to it renumbers every category after it, and
every stored layer would silently start grading something else. A name that
no longer exists is simply not found and the layer reads as stale, which is
the failure that announces itself.

Staleness is otherwise identical to a subject's: the coverage buffer is in
the session, never the sidecar, so a signature from another run points at
pixels that were never computed.

## Two tests, and the second one caught a real shape

Round-tripping the name matters more than usual here, because the whole
argument for storing a name instead of an index is worthless if the sidecar
is what drops it.

The multi-word case is the one worth having: `category = swimming pool` is
written on one line, and a reader splitting on whitespace would have
truncated it to a category no model has — a layer that silently masks
nothing. `category` is also its own key rather than a reuse of `class`,
because a file conflating them would round-trip a subject into a category.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-pipeline/src/mask.rs

@@ -532,6 +532,38 @@ pub enum MaskSource {
         score: f32,
     },
 
+    /// Every pixel of one photographic category, from the scene model.
+    ///
+    /// The counterpart to [`Self::Subject`], and the difference is the whole
+    /// reason both exist. A subject is *one* instance — this dog, not that one
+    /// — found by a COCO-trained instance model. A category is *all* the sky,
+    /// or all the foliage, from an ADE20K-trained semantic model that has no
+    /// notion of instances at all (docs/segmentation.md §16).
+    ///
+    /// So this is what a global grade attaches to: lift the sky, desaturate
+    /// the vegetation, warm the architecture. Asking it for "that person
+    /// rather than the other two" is a category error — the model merged them
+    /// before the mask ever existed, and [`Self::Subject`] is the source for
+    /// that question.
+    ///
+    /// Stored as identity like a subject, and for the same reason: the
+    /// coverage is megabytes and is reproducible from the same model over the
+    /// same image.
+    Category {
+        /// Which segmentation run produced it, so a layer can tell whether
+        /// the name below still refers to something that was computed.
+        signature: u64,
+        /// The category name from `models/scene/categories.txt` — "sky",
+        /// "vegetation".
+        ///
+        /// A name rather than an index because the descriptor is editable: a
+        /// category added to it would silently renumber every layer stored
+        /// against an index, and the failure would be a mask quietly grading
+        /// the wrong thing. A name that no longer exists is simply not found,
+        /// and the layer reads as stale.
+        name: String,
+    },
+
     /// A linear gradient — the graduated-filter mask.
     ///
     /// Geometry is in **normalised source coordinates**, so it survives a crop,
@@ -596,6 +628,7 @@ impl MaskSource {
         match self {
             Self::Regions { .. } => "regions",
             Self::Subject { .. } => "subject",
+            Self::Category { .. } => "category",
             Self::Linear { .. } => "linear",
             Self::Radial { .. } => "radial",
             Self::Brush { .. } => "brush",
@@ -825,9 +858,9 @@ impl MaskLayer {
     /// should offer to recompute rather than render it.
     pub fn is_stale(&self, current: u64) -> bool {
         match self.source {
-            MaskSource::Regions { signature, .. } | MaskSource::Subject { signature, .. } => {
+            MaskSource::Regions { signature, .. }
-                signature != current
+            | MaskSource::Subject { signature, .. }
-            }
+            | MaskSource::Category { signature, .. } => signature != current,
             // A gradient is geometry in normalised coordinates. It means the
             // same thing whatever was or was not detected, so nothing about a
             // new run can invalidate it. Painted strokes are the same: they are

core/dr-pipeline/src/sidecar.rs

@@ -971,6 +971,10 @@ fn write_mask(out: &mut String, version: &str, layer: &MaskLayer) {
             let _ = writeln!(out, "class = {class}");
             let _ = writeln!(out, "score = {}", format_value(*score));
         }
+        MaskSource::Category { signature, name } => {
+            let _ = writeln!(out, "signature = {signature}");
+            let _ = writeln!(out, "category = {name}");
+        }
         MaskSource::Linear {
             centre,
             angle,
@@ -1124,6 +1128,7 @@ struct PartialMask {
     ids: Vec<u32>,
     index: u32,
     class: String,
+    category: String,
     score: f32,
     centre: (f32, f32),
     radii: (f32, f32),
@@ -1154,6 +1159,7 @@ impl PartialMask {
             level: 0,
             ids: Vec::new(),
             index: 0,
+            category: String::new(),
             class: String::new(),
             score: 0.0,
             centre: (0.5, 0.5),
@@ -1191,6 +1197,11 @@ impl PartialMask {
                 self.ids.sort_unstable();
                 self.ids.dedup();
             }
+            // A category's own key rather than reusing `class`: both name a
+            // thing the mask covers, but one is a COCO instance's label and
+            // the other an entry in the scene descriptor, and a file that
+            // conflated them would round-trip a subject into a category.
+            "category" => self.category = value.to_string(),
             "index" => self.index = value.parse().unwrap_or(0),
             "class" => self.class = value.to_string(),
             "score" => self.score = value.parse().unwrap_or(0.0),
@@ -1257,6 +1268,10 @@ impl PartialMask {
                 class: self.class,
                 score: self.score,
             },
+            "category" => MaskSource::Category {
+                signature: self.signature,
+                name: self.category,
+            },
             "linear" => MaskSource::Linear {
                 centre: self.centre,
                 angle: self.angle,

core/dr-pipeline/tests/mask_sidecar.rs

@@ -616,3 +616,64 @@ fn show_a_sidecar() {
 
     println!("\n{}", sidecar.to_text());
 }
+
+/// A category mask is a name plus a signature, and both have to survive.
+///
+/// The name especially. `MaskSource::Category` stores one rather than an index
+/// precisely so that editing `models/scene/categories.txt` cannot repoint a
+/// stored layer — and that reasoning is worth nothing if the name is what the
+/// sidecar drops.
+#[test]
+fn category_masks_keep_their_name() {
+    let mut graph = EditGraph::default_chain();
+
+    let mut sky = MaskLayer::new(
+        "m1",
+        MaskSource::Category {
+            signature: 0x5EED_1234,
+            name: "sky".into(),
+        },
+    );
+    sky.name = "sky".into();
+    sky.feather = 0.03;
+    sky.set_param("exposure", ParamId("exposure"), 0.5);
+    graph.masks_mut().push(sky);
+
+    // A multi-word name too: the writer emits `category = swimming pool` on one
+    // line, and a reader that split on whitespace would silently truncate it to
+    // a category no model has.
+    let mut pool = MaskLayer::new(
+        "m2",
+        MaskSource::Category {
+            signature: 0x5EED_1234,
+            name: "swimming pool".into(),
+        },
+    );
+    pool.set_param("saturation", ParamId("saturation"), -30.0);
+    graph.masks_mut().push(pool);
+
+    let restored = round_trip(&graph);
+    let layers = restored.masks().layers();
+    assert_eq!(layers.len(), 2);
+    assert_eq!(layers[0].source, graph.masks().layers()[0].source);
+    assert_eq!(layers[1].source, graph.masks().layers()[1].source);
+    assert!((layers[0].feather - 0.03).abs() < 1e-6);
+}
+
+/// A category is stale when its run is, exactly as a subject is.
+///
+/// The coverage buffer lives in the session, not the sidecar, so a layer whose
+/// signature no longer matches is pointing at pixels that were never computed.
+/// Rendering it anyway would mask nothing and read as a broken adjustment.
+#[test]
+fn a_category_from_another_run_is_stale() {
+    let layer = MaskLayer::new(
+        "m1",
+        MaskSource::Category {
+            signature: 1,
+            name: "sky".into(),
+        },
+    );
+    assert!(layer.is_stale(2), "a different run must invalidate it");
+    assert!(!layer.is_stale(1), "the run it was built against must not");
+}