Masonry goes column-major — cards read top-down, then across

Replaces the pathfinder-inherited round-robin deal (child i -> column i % C)
with contiguous column segments: base = n/C, the first n%C columns take one
more, and logical order runs down each column before crossing to the next.
Only the geometric mapping changes -- ranks, selection flatten, and VoiceOver
order are untouched, and MasonryPlacement stays the single placement function
both the Layout and the drop model replay.

Why: an insertion under round-robin shifted every later card across columns;
under the column-major deal later cards slide within their column and at most
one card crosses each boundary, so the drag reflow is far calmer. Drop-slot
math gets simpler too -- a column's cards are one contiguous range, a
non-final column's tail is now a genuine mid-list position, and only the last
column's tail means append.

DropSlotMathTests recomputed and extended (46 -> 50): the uneven-fill deal,
boundary positions, the shared tail/head boundary index, and a placement/
drop-model shadow-agreement check. DRAG-REORDER.md and DESIGN/10 amendments
are listed for ratification, deliberately not edited here.

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

Kanban/UI/Board/LaneView.swift

@@ -704,20 +704,18 @@ struct LaneView: View {
                     // holds identically under Reduce Motion: a transition that does not fire has no
                     // variant to choose between.
                     .transition(Motion.cardTransition(reduced: reduceMotion))
-                    // **VoiceOver reads the masonry by `order`, not by column** — 10-accessibility.md
-                    // ▸ Logical order, not masonry position (decided): "within a wide lane,
-                    // VoiceOver reads cards by `order` — the interior grid columns are presentation
-                    // only. This deliberately diverges from on-screen geometry."
+                    // **VoiceOver reads the masonry by `order`, not by drawn position** —
+                    // 10-accessibility.md ▸ Logical order, not masonry position (decided).
                     //
-                    // The divergence is real and it is why an explicit priority is needed at all:
-                    // `MasonryLayout` assigns child `i` to column `i % columns`, so in a 3-unit lane
-                    // the second card by `order` is drawn to the *right* of the first, not below it
-                    // — and an accessibility tree sorted by geometry (which is what a container does
-                    // without this) would read the board column-major: 1, 4, 7, 2, 5, 8 …, an order
-                    // that exists nowhere in the model, on disk, or in the keyboard grammar.
-                    // Priority descends with the slot index, so the highest reads first and the list
-                    // is exactly `slots` — the same sequence the masonry is handed and the same one
-                    // `SelectionGrammar` flattens.
+                    // The divergence narrowed when the masonry went column-major — walking down
+                    // one column now *is* consecutive `order` — but it is still real, and it is
+                    // why an explicit priority is needed at all: a geometry-sorted accessibility
+                    // tree (which is what a container does without this) sweeps in reading order,
+                    // left-to-right then down, which over a column-major grid interleaves the
+                    // columns: 1, 4, 7, 2, 5, 8 …, an order that exists nowhere in the model, on
+                    // disk, or in the keyboard grammar. Priority descends with the slot index, so
+                    // the highest reads first and the list is exactly `slots` — the same sequence
+                    // the masonry is handed and the same one `SelectionGrammar` flattens.
                     //
                     // The drag shadows are inert here: `DragShadow` hides itself from the tree, and
                     // a slot that is not an element consumes no priority.