Build the drop-slot model and the drop commits — drag & drop, first half

The pathfinder's drag-reorder model, ported and generalized (DRAG-REORDER.md
travels with it, rewritten for lanes, the interior masonry, multi-drag,
cross-board sessions, the re-grounding trio, and the committed-overlay hold):

- DropSlotMath — resting-layout zones from analytic lane arithmetic and the
  pure masonry placement (MasonryLayout now lays out through the same
  MasonryPlacement the drag reads, so geometry cannot drift), span-capped
  triggers sized to the dragged run's future footprint, hysteresis holds with
  the fresh-entry fallback, boundary ties, own-slot no-ops; nil means hold.
- DragAutoScrollMath — the activation bands and velocity ramp, pure.
- The drop commits, one performWrite bracket each: moveCards/copyCards within
  a board (insertion ranks touch only the dragged cards; renumber fallback);
  receiveCards/receiveLanes/receiveRestoredCards on the destination store for
  cross-board copy and ⌘-move with the import-boundary remint, lane copies
  stripping tombstoned cards while moves carry them; restoreByDrag is now
  positional, writing order only when the drop names a new one.

Gestures, sessions, previews, and delegates are the second half.

773 unit tests (87 new since the keyboard grammar).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

KanbanTests/BoardWriterTests.swift

@@ -1571,6 +1571,94 @@ struct BoardWriterCopyTests {
     }
 }
 
+// MARK: - Stripping a copied lane's tombstones
+
+/// `BoardWriter.stripTombstonedChildren` — the tail of a lane copy (04-interactions.md ▸ Drag and
+/// drop: "A lane copy **strips tombstoned cards**"). `copyItem` copies the tree verbatim by
+/// design, so the strip is the line after it rather than a filter inside it.
+struct BoardWriterStripTombstonesTests {
+
+    /// A tombstoned card, as an agent or a delete leaves it.
+    private static func tombstone(order: String, title: String) -> String {
+        "---\nschema: 1\ntitle: \(title)\norder: \(order)\ndeleted: 2026-03-03T09:00:00Z\n---\n\(title) body.\n"
+    }
+
+    @Test func onlyTheTombstonedChildrenAreRemoved() throws {
+        let fixture = try WriterFixture()
+        defer { fixture.tearDown() }
+        try fixture.item("A.kanban", Item.board)
+        let lane = try fixture.item("A.kanban/\(Ident.lane1)", Item.rich(order: "1024", title: "Todo"))
+        try fixture.item("A.kanban/\(Ident.lane1)/\(Ident.card1)", Item.rich(order: "1024", title: "Live"))
+        try fixture.item("A.kanban/\(Ident.lane1)/\(Ident.card2)",
+                         Self.tombstone(order: "2048", title: "Trashed"))
+        try fixture.item("A.kanban/\(Ident.lane1)/\(Ident.card3)", Item.rich(order: "3072", title: "Also live"))
+        let live = try fixture.indexText("A.kanban/\(Ident.lane1)/\(Ident.card1)")
+
+        let removed = try BoardWriter.stripTombstonedChildren(of: lane)
+
+        #expect(removed == [ItemID(rawValue: Ident.card2)])
+        #expect(!fixture.exists("A.kanban/\(Ident.lane1)/\(Ident.card2)"))
+        // Removed, never tombstoned, and the survivors are not rewritten on the way past.
+        #expect(fixture.exists("A.kanban/\(Ident.lane1)/\(Ident.card1)"))
+        #expect(fixture.exists("A.kanban/\(Ident.lane1)/\(Ident.card3)"))
+        #expect(try fixture.indexText("A.kanban/\(Ident.lane1)/\(Ident.card1)") == live)
+        #expect(try BoardLoader.load(boardRoot: fixture.url("A.kanban")).warnings.isEmpty)
+    }
+
+    @Test func aWholeTombstonedFolderGoesWithItsAttachments() throws {
+        let fixture = try WriterFixture()
+        defer { fixture.tearDown() }
+        try fixture.item("A.kanban", Item.board)
+        let lane = try fixture.item("A.kanban/\(Ident.lane1)", Item.rich(order: "1024", title: "Todo"))
+        try fixture.item("A.kanban/\(Ident.lane1)/\(Ident.card1)",
+                         Self.tombstone(order: "1024", title: "Trashed"))
+        try fixture.file("A.kanban/\(Ident.lane1)/\(Ident.card1)/attachments/shot.png", Data([0x89, 0x50]))
+
+        _ = try BoardWriter.stripTombstonedChildren(of: lane)
+
+        #expect(!fixture.exists("A.kanban/\(Ident.lane1)/\(Ident.card1)"))
+        #expect(try fixture.entryNames("A.kanban/\(Ident.lane1)") == ["index.md"])
+    }
+
+    @Test func nonUUIDStraysAndUnreadableChildrenAreLeftAlone() throws {
+        let fixture = try WriterFixture()
+        defer { fixture.tearDown() }
+        try fixture.item("A.kanban", Item.board)
+        let lane = try fixture.item("A.kanban/\(Ident.lane1)", Item.rich(order: "1024", title: "Todo"))
+        // A stray is not a level at all; a UUID-shaped folder with no `index.md` cannot be asked
+        // the liveness question, and the conservative direction is to keep it.
+        try fixture.file("A.kanban/\(Ident.lane1)/notes/scratch.txt", Data("hand-written\n".utf8))
+        try FileManager.default.createDirectory(at: fixture.url("A.kanban/\(Ident.lane1)/\(Ident.indexless)"),
+                                                withIntermediateDirectories: true)
+
+        let removed = try BoardWriter.stripTombstonedChildren(of: lane)
+
+        #expect(removed.isEmpty)
+        #expect(fixture.exists("A.kanban/\(Ident.lane1)/notes/scratch.txt"))
+        #expect(fixture.exists("A.kanban/\(Ident.lane1)/\(Ident.indexless)"))
+    }
+
+    @Test func aLaneWithNothingTombstonedIsUntouched() throws {
+        let fixture = try WriterFixture()
+        defer { fixture.tearDown() }
+        try fixture.item("A.kanban", Item.board)
+        let lane = try fixture.item("A.kanban/\(Ident.lane1)", Item.rich(order: "1024", title: "Todo"))
+        try fixture.item("A.kanban/\(Ident.lane1)/\(Ident.card1)", Item.rich(order: "1024", title: "Live"))
+
+        #expect(try BoardWriter.stripTombstonedChildren(of: lane).isEmpty)
+        #expect(try fixture.entryNames("A.kanban/\(Ident.lane1)").sorted() == [Ident.card1, "index.md"].sorted())
+    }
+
+    @Test func aMissingFolderIsALoudErrorNamingTheCopy() throws {
+        let fixture = try WriterFixture()
+        defer { fixture.tearDown() }
+
+        let error = writeFailure { _ = try BoardWriter.stripTombstonedChildren(of: fixture.url("A.kanban/\(Ident.lane1)")) }
+        // The user pressed nothing called "delete": a failure here must say the copy failed.
+        #expect(error?.operation == .copy(title: nil))
+    }
+}
+
 // MARK: - Delete / Restore
 
 /// `BoardWriter.deleteItem`/`restoreItem` — the tombstone half of 01-storage-format.md §