Swatches: swatch.library.save counts only the swatches the file holds

swatch.library.save replied with the number of solid colors and
gradients it was asked to save, including the gradients the format
leaves out. For a new document saved as .ase, count was 50 and the file
held 46 colors. The .gpl and .ase writers keep solid colors only, and
the CSS writer leaves freeform gradients out.

count is now the number of swatches written. The new
PaletteFormat::holds returns whether a file of a format keeps a swatch
with a given paint, and swatch.library.save counts the swatches for
which it returns true. The reply has the same fields as before.

Tests: after a freeform gradient is added to a new document's swatches,
the test checks that count has these values: 51 for .vcswatches, 46 for
.gpl and .ase, and 50 for CSS. Each file holds that many swatches.

Docs: docs/mcp.md.

crates/color/src/palette_io.rs

@@ -57,6 +57,19 @@ impl PaletteFormat {
     pub fn binary(self) -> bool {
         self == PaletteFormat::Ase
     }
+    /// Does [`write`] keep a swatch with `paint` in a file of this format? None and patterns are
+    /// never written; `.gpl` and `.ase` hold solid colors only, and CSS holds no freeform gradients.
+    pub fn holds(self, paint: &Paint) -> bool {
+        match paint {
+            Paint::Solid { .. } => true,
+            Paint::Gradient(g) => match self {
+                PaletteFormat::Native => true,
+                PaletteFormat::Css => g.gradient.kind != GradientKind::Freeform,
+                PaletteFormat::Gpl | PaletteFormat::Ase => false,
+            },
+            Paint::None | Paint::Pattern { .. } => false,
+        }
+    }
 }
 
 /// The native file: a header around the library.
@@ -792,6 +805,23 @@ mod tests {
         assert!(read(&text, "x").is_err(), "CSS is written only");
     }
 
+    #[test]
+    fn holds_counts_the_swatches_each_format_writes() {
+        // `sample()` holds the spot color Ink, the linear gradient Fade, the pattern Tiles and two solid colors in a group.
+        let mut lib = sample();
+        let Paint::Gradient(mut mesh) = lib.swatches[1].paint.clone() else { panic!("Fade is a gradient") };
+        mesh.gradient.kind = GradientKind::Freeform;
+        lib.swatches.push(Swatch { name: "Mesh".into(), paint: Paint::Gradient(mesh), global: false, spot: false });
+        lib.swatches.push(Swatch { name: "[None]".into(), paint: Paint::None, global: false, spot: false });
+        let held = |f: PaletteFormat| lib.iter().filter(|w| f.holds(&w.paint)).count();
+        assert_eq!(PaletteFormat::ALL.map(held), [5, 3, 3, 4], "vcswatches, gpl, ase, css");
+        for f in [PaletteFormat::Native, PaletteFormat::Gpl, PaletteFormat::Ase] {
+            assert_eq!(read_bytes(&write(&lib, f).unwrap(), "x").unwrap().len(), held(f), "{f:?}");
+        }
+        let css = written(&lib, PaletteFormat::Css);
+        assert_eq!(css.lines().filter(|l| l.starts_with("  --")).count(), held(PaletteFormat::Css), "one property each: {css}");
+    }
+
     /// A swatch exchange file of `blocks` (type, body).
     fn ase(blocks: &[(u16, Vec<u8>)]) -> Vec<u8> {
         let mut out = b"ASEF\0\x01\0\0".to_vec();
@@ -1088,5 +1118,15 @@ mod tests {
                 prop_assert!(same_swatches(&g.swatches, &ase_expected(&want.swatches)), "{}: {:?}", g.name, g.swatches);
             }
         }
+
+        /// A written `.vcswatches`, `.gpl` or `.ase` file reads back with as many swatches as
+        /// [`PaletteFormat::holds`] counts.
+        #[test]
+        fn written_files_read_back_with_the_swatches_holds_counts(lib in arb_library()) {
+            for f in [PaletteFormat::Native, PaletteFormat::Gpl, PaletteFormat::Ase] {
+                let back = read_bytes(&write(&lib, f).unwrap(), &lib.name).unwrap();
+                prop_assert_eq!(back.len(), lib.iter().filter(|w| f.holds(&w.paint)).count(), "{:?}", f);
+            }
+        }
     }
 }