The pickup seeds its own slot — the lift and the shadow become one transaction

"At drag start it replaces the item's original space" is now true from the
very first frame: begin takes the run's own resting slot as a seed, set in
the same transaction that lifts the run out. Without it the vacated gap
closed un-animated and sprang back open at the first dropUpdated — a
shuffle carrying no information. The seed bypasses propose deliberately (a
pickup is not a new landing spot, so no alignment tick), and the first real
sample's re-propose of the same slot is the early-out's ordinary silence. A
⌥-pickup seeds nothing — a copy's resting layout keeps the originals in
place, so there is no vacated space to hold. Card pickups compute the seed
through the same LaneView.rendered rules the retargets count in; lane
pickups seed their strip slot the same way. DRAG-REORDER.md's own-slot
paragraph records the rule, and SeededPickupTests pins all four corners.

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

DRAG-REORDER.md

@@ -34,7 +34,7 @@ Lanes get this for free and the invariant is worth stating: a within-board lane
 Two stability rules on top:
 
 - **Exact-boundary tie** — a cursor resting on a boundary pixel keeps the current proposal when it adjoins that boundary; the shadow can never oscillate on a single pixel.
-- **Own-slot pickup** — zones derive from the resting layout, so picking an item up over its original spot proposes its own slot: a no-op, no reflow, and the store's commits refuse to write for it.
+- **Own-slot pickup** — zones derive from the resting layout, so picking an item up over its original spot proposes its own slot: a no-op, no reflow, and the store's commits refuse to write for it. The own slot is also **seeded at `begin`** (`DragSession.begin`'s `seed`), in the same transaction that lifts the run out, so "at drag start it replaces the item's original space" is true from the very first frame: without the seed the vacated gap closes un-animated and springs back open at the first `dropUpdated` — a shuffle carrying no information. The seed bypasses `propose` (a pickup is not a new landing spot, so no alignment tick), and the first real sample's re-propose of the same slot is the early-out's ordinary silence. A ⌥-pickup seeds nothing — a copy's resting layout keeps the originals in place, so there is no vacated space to hold, and the first sample answers as it always did.
 
 ## The lane strip's resting layout is arithmetic