Generated and pasted board backgrounds move into .backgrounds/ — the board root stops collecting the app's own pictures

New app-written background images (Theme tab ▸ Pattern, and Edit ▸ Paste
as Board Background) now land in a hidden `.backgrounds/` folder at
board root instead of beside index.md, matching the `.trash/` app-managed
pattern. `background.image` stores the qualified relative reference
(`.backgrounds/facets.png`); the resolver needed no change at all, since
it already accepted any relative path inside the board root — the same
mechanism that already resolved `art/backdrops/sunset.png` resolves the
new location for free. `BoardWriter.writeBoardImage` now creates its
destination folder if missing, since `.backgrounds/` won't exist until a
board's first generated or pasted background.

The Finder collision-ladder (`BoardStore.boardImageName`) is rescoped to
`.backgrounds/`'s own contents, and its overwrite-in-place check now
recognizes only the qualified form as "ours" — a legacy bare
`background.image: facets.png` from before this change is read as a
foreign reference rather than migrated, so a regeneration writes a fresh
`.backgrounds/` file and orphans the old one in place, per the no-migration
ruling. The Theme tab's Pattern/Solid mode-detection was updated to
recognize both the legacy and current spellings as the generator's own
output.

The board loader needed no change: `.backgrounds/` is a hidden,
non-UUID-shaped name, and `.skipsHiddenFiles` already keeps every hidden
entry off the lane walk before any name-based exclusion is consulted —
pinned with a new loader test. Deliberately did not add `.backgrounds` to
IntegrityRules' claimed-name/squatter-displacement table: that table
mirrors a specific existing DESIGN ruling this card doesn't amend.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

KanbanTests/PasteImageTests.swift

@@ -665,7 +665,7 @@ struct PasteBoardBackgroundTests {
         try FrontmatterDocument.parse(fixture.indexText(""))
     }
 
-    @Test("The picture lands in the board folder and `background.image` names it")
+    @Test("The picture lands in .backgrounds/ and `background.image` names it")
     func thePictureLands() throws {
         let harness = try makeClipboardHarness()
         defer { harness.tearDown() }
@@ -674,8 +674,8 @@ struct PasteBoardBackgroundTests {
 
         #expect(harness.clipboard.pasteBoardBackground(into: harness.store))
 
-        #expect(try harness.fixture.data("Pasted Background.png") == png)
-        #expect(try background(harness.fixture).backgroundImage.value == "Pasted Background.png")
+        #expect(try harness.fixture.data(".backgrounds/Pasted Background.png") == png)
+        #expect(try background(harness.fixture).backgroundImage.value == ".backgrounds/Pasted Background.png")
         #expect(harness.store.banners.oneShots.isEmpty)
     }
 
@@ -693,7 +693,7 @@ struct PasteBoardBackgroundTests {
 
         let document = try background(harness.fixture)
         #expect(document.background.value == "#112233")
-        #expect(document.backgroundImage.value == "Pasted Background.png")
+        #expect(document.backgroundImage.value == ".backgrounds/Pasted Background.png")
     }
 
     /// The generator's overwrite-in-place rule, inherited: re-pasting must not leave a folder full of
@@ -710,24 +710,25 @@ struct PasteBoardBackgroundTests {
         harness.pasteboard.seed([(UTType.png.identifier, second)])
         #expect(harness.clipboard.pasteBoardBackground(into: harness.store))
 
-        #expect(try harness.fixture.data("Pasted Background.png") == second)
-        #expect(!harness.fixture.exists("Pasted Background 2.png"))
+        #expect(try harness.fixture.data(".backgrounds/Pasted Background.png") == second)
+        #expect(!harness.fixture.exists(".backgrounds/Pasted Background 2.png"))
     }
 
-    /// A hand-placed file of that name is the user's, and is never written through — the ladder's
-    /// rule, the same one the generator follows.
+    /// A hand-placed file of that name **inside `.backgrounds/`** is the user's, and is never written
+    /// through — the ladder's rule, the same one the generator follows. A file of that name at board
+    /// root would not collide at all any more: it simply is not where this write ever looks.
     @Test("A file already holding the name is stepped around")
     func anExistingNameIsRespected() throws {
         let harness = try makeClipboardHarness()
         defer { harness.tearDown() }
         let mine = Data("hand placed".utf8)
-        try harness.fixture.file("Pasted Background.png", mine)
+        try harness.fixture.file(".backgrounds/Pasted Background.png", mine)
         harness.pasteboard.seed([(UTType.png.identifier, encodedImage(.png))])
 
         #expect(harness.clipboard.pasteBoardBackground(into: harness.store))
 
-        #expect(try harness.fixture.data("Pasted Background.png") == mine)
-        #expect(try background(harness.fixture).backgroundImage.value == "Pasted Background 2.png")
+        #expect(try harness.fixture.data(".backgrounds/Pasted Background.png") == mine)
+        #expect(try background(harness.fixture).backgroundImage.value == ".backgrounds/Pasted Background 2.png")
     }
 
     /// The two producers must not read each other's echo: a paste landing inside the reroll's window
@@ -743,8 +744,8 @@ struct PasteBoardBackgroundTests {
 
         #expect(harness.clipboard.pasteBoardBackground(into: harness.store))
 
-        #expect(try harness.fixture.data(FacetsGenerator.fileName) == generated, "untouched")
-        #expect(try background(harness.fixture).backgroundImage.value == "Pasted Background.png")
+        #expect(try harness.fixture.data(".backgrounds/\(FacetsGenerator.fileName)") == generated, "untouched")
+        #expect(try background(harness.fixture).backgroundImage.value == ".backgrounds/Pasted Background.png")
     }
 
     @Test("⌘Z puts the image subkey back and leaves the colour alone")
@@ -757,17 +758,17 @@ struct PasteBoardBackgroundTests {
 
         #expect(store.applyPastedBackground(data: encodedImage(.png), fileExtension: "png"))
         #expect(try FrontmatterDocument.parse(fixture.indexText("")).backgroundImage.value
-            == "Pasted Background.png")
+            == ".backgrounds/Pasted Background.png")
         #expect(history.undoActionName == "Restyle Board")
 
         history.undo()
         #expect(try FrontmatterDocument.parse(fixture.indexText("")).backgroundImage.isMissing)
         // The file survives the undo — "the undo restores the field, not the bytes".
-        #expect(fixture.exists("Pasted Background.png"))
+        #expect(fixture.exists(".backgrounds/Pasted Background.png"))
 
         history.redo()
         #expect(try FrontmatterDocument.parse(fixture.indexText("")).backgroundImage.value
-            == "Pasted Background.png")
+            == ".backgrounds/Pasted Background.png")
     }
 }